Lithofacies Types Research Articles

Sedimentological Controls on the Reservoir Characteristics of the Mid-Triassic Tredian Formation in the Salt and Trans-Indus Surghar Ranges, Pakistan: Integration of Outcrop, Petrographic, and SEM Analyses

The current study uses an integrated lithofacies, optical microscopy, and scanning electron microscopy (SEM) analysis to investigate the sedimentary processes, depositional architecture, and reservoir rock potential of the Tredian Formation’s (Mid-Triassic) mixed siliciclastic and carbonate succession in the Salt and Trans-Indus Ranges. The formation has been divided litho-stratigraphically into two components: the lower Landa Member, which consists of fine-grained sandstone and shale, and the upper Khatkiara Member, which consists of coarse-grained sandstone. Based on sedimentary structures and lithology, four distinct types of lithofacies are identified. Two lithofacies representing sandstones interbedded with shale (LF1) and thick-bedded sandstone (LF2) lithofacies suggestive of fluvio-deltaic settings are among them. Another two lithofacies of thin-bedded sandstone (LF3) and dolomite (LF4) suggest a tidal flat depositional environment, correspondingly. The petrographic examination of the Tredian sandstones indicates a lithology ranging from sub-feldspathic arenite to feldspathic arenite with moderate packing. The presence of primary calcite cement, silica cement, and iron oxide/hydroxide cements were shown by the diagenetic investigation, which was supported by SEM studies. In addition, secondary cements include ferroan-dolomite, chlorite, and illite, which is linked with chemical alteration of unstable grains. The paragenetic sequence depicts the diagenetic evolution of the Tredian sandstone from early to late diagenetic phases. The reservoir quality of the LF1 and LF4 lithofacies has been destroyed by early-stage calcite cementation, but the lithofacies LF2 and LF3 have a strong reservoir potential owing to the scarcity of calcite cement, dissolution of unstable feldspar grains, and grain fracture.

Pore structure characteristics of different lithofacies of the Longmaxi shale, Western Hunan‐Hubei Region, China: Implications for reservoir quality prediction

Lithofacies and micropore characteristics of shale reservoir directly affect the exploration and development of natural gas. To determine the development potential of shales of the Longmaxi Formation in the Western Hunan‐Hubei Region, the lithofacies types and micropore structure characteristics of the Longmaxi Formation shales have been systematically studied in this paper, based on ordinary thin sections, argon ion polishing–scanning electron microscopy, physical property testing, x‐ray diffraction whole‐rock and clay mineral analysis, isothermal nitrogen, and carbon dioxide adsorptions. According to the principle of ‘laminar structure + total organic carbon (TOC) + mineral composition of felsic, clay, and carbonate minerals’, eight types of shale lithofacies were identified. The effective porosity of the Longmaxi shale ranges from 1.8% to 14.4%, with a peak range from 3% to 6%, while the permeability ranges from 0.001 to 1.27 mD, with an average of 0.106 mD. Combined with the isothermal adsorption experiments, the proportion of mesopores is the largest in the Longmaxi shale, followed by macropores and micropores. Moreover, the pore diameters of mesopores are concentrated between 3 and 10 nm, while those of micropores are concentrated between 0.4 and 0.9 nm. The pore volume of shale is most developed when the content of brittle felsic minerals is 50% ~ 75% and the content of clay minerals is 25% ~ 50%, and the TOC content is positively correlated with the pore volume and specific surface area. There are significant differences in the pore development characteristics of different lithofacies. The shale reservoir space analysis indicates that micropores are controlled by TOC content and clay minerals, while the macropores are controlled by depositional intergranular pores, TOC content, and felsic content, as well as diagenetic pyrite mould pores, microfractures pores, and intercrystalline pores. On the other hand, mesopores are related to a mix of depositional and diagenetic factors with dominant contribution of intergranular pores, intercrystalline pores, organic pores, and intergranular shrinkage fractures pores. It is found that the pore development of massive organic‐rich siliceous shale lithofacies has the best reservoir quality, including well‐connected intergranular pores, intercrystalline pores, organic pores, and shrinkage fractures. Therefore, the massive organic‐rich siliceous shale lithofacies is the dominant fine‐grained reservoir lithofacies in the studied area. Based on understating the pore characteristics of different lithofacies, this study can provide a scientific basis for the prediction of favourable sweet spots of shale gas reservoir in the Longmaxi Formation, western Hunan‐Hubei Region.

Evaluating Shale Gas Sweet Spots of the Jurassic Da'anzhai Member in the Yuanba Area, Sichuan Basin

Terrestrial shale strata generally has frequent and strong heterogeneity characteristics, thus affecting shale gas enrichment and target selection. In this study, samples from the Da’anzhai Member of Well A in Yuanba area have performed a series of experiments including total organic carbon ( TOC) content measurement, rock pyrolysis, organic petrology, vitrinite reflectance ( Ro), whole-rock mineral composition, thin section analysis, focused ion beam scanning electron microscope, high-pressure Hg injection, nitrogen (N2) adsorption, and methane (CH4) isothermal adsorption. The results show that: (1) Da'anzhai (J1da) Member is a set of mixed sediments with various lithology, which consists of black shale and well-developed siltstone and carbonate rocks interlayers. The second section of the Da'anzhai Member (J1da2) has relatively greater organic matter abundance (1.24% TOC), better organic matter type dominated by type III and II, as well as higher maturity (1.67% Ro), which is the most favorable section of J1da. (2) J1da2 shale strata can be divided into three shale lithofacies types, two interlayer lithofacies types, and three macro-lithofacies assemblage types. Among them, mudstone–limestone lithofacies assemblage (type AB-I) has preferable hydrocarbon generation conditions, better reservoir properties, and more developed shell limestone interlayers with better fracturing performance, is thought the most favorable interval. (3) The optimal interval and horizontal target of J1da2 are further selected based on the source–reservoir coupling relationship, corresponding to the depth is 3909–3914 m, which imply the optimal horizontal target window for Well A. This study has application values not only for the identification of “sweet intervals” and well location deployment of J1da but also supports the later systematic studies of shale gas accumulation mechanism and enrichment regularity in Yuanba area.

Forecast of lacustrine shale lithofacies types in continental rift basins based on machine learning: A case study from Dongying Sag, Jiyang Depression, Bohai Bay Basin, China

Lacustrine shale in continental rift basins is complex and features a variety of mineralogical compositions and microstructures. The lithofacies type of shale, mainly determined by mineralogical composition and microstructure, is the most critical factor controlling the quality of shale oil reservoirs. Conventional geophysical methods cannot accurately forecast lacustrine shale lithofacies types, thus restricting the progress of shale oil exploration and development. Considering the lacustrine shale in the upper Es4 member of the Dongying Sag in the Jiyang Depression, Bohai Bay Basin, China, as the research object, the lithofacies type was forecast based on two machine learning methods: support vector machine (SVM) and extreme gradient boosting (XGBoost). To improve the forecast accuracy, we applied the following approaches: first, using core and thin section analyses of consecutively cored wells, the lithofacies were finely reclassified into 22 types according to mineralogical composition and microstructure, and the vertical change of lithofacies types was obtained. Second, in addition to commonly used well logging data, paleoenvironment parameter data (Rb/Sr ratio, paleoclimate parameter; Sr %, paleosalinity parameter; Ti %, paleoprovenance parameter; Fe/Mn ratio, paleo-water depth parameter; P/Ti ratio, paleoproductivity parameter) were applied to the forecast. Third, two sample extraction modes, namely, curve shape-to-points and point-to-point, were used in the machine learning process. Finally, the lithofacies type forecast was carried out under six different conditions. In the condition of selecting the curved shape-to-point sample extraction mode and inputting both well logging and paleoenvironment parameter data, the SVM method achieved the highest average forecast accuracy for all lithofacies types, reaching 68%, as well as the highest average forecast accuracy for favorable lithofacies types at 98%. The forecast accuracy for all lithofacies types improved by 7%–28% by using both well logging and paleoenvironment parameter data rather than using one or the other, and was 7%–8% higher by using the curve shape-to-point sample extraction mode compared to the point-to-point sample extraction mode. In addition, the learning sample quantity and data value overlap of different lithofacies types affected the forecast accuracy. The results of our study confirm that machine learning is an effective solution to forecast lacustrine shale lithofacies. When adopting machine learning methods, increasing the learning sample quantity (>45 groups), selecting the curve shape-to-point sample extraction mode, and using both well logging and paleoenvironment parameter data are effective ways to improve the forecast accuracy of lacustrine shale lithofacies types. The method and results of this study provide guidance to accurately forecast the lacustrine shale lithofacies types in new shale oil wells and will promote the harvest of lacustrine shale oil globally.

Reservoir characteristics of marine–continental transitional shale and gas-bearing mechanism:Understanding based on comparison with marine shale reservoir

Transitional shale gas layers are widely distributed in China, but no significant exploration breakthrough has been made so far. This paper investigates and analyzes the characteristics and gas-bearing mechanisms of transitional shale gas reservoirs in detail, aiming to clarify the shale gas accumulation mechanism of transitional shale and provide theoretical support for the selection of favorable intervals. The transitional shale gas layer is characterized by a thin single-layer thickness, rapid lithological change, low brittle mineral content, and poor kerogen type. The lack of organic matter (OM) sponge pore development process from the oil-generation stage results in limited numbers of OM nanometer-scale pores. Shale pore space is dominated by pores and fractures related to clay minerals. The measured gas content is well consistent with the theoretically calculated gas content for marine organic-rich shales. However, the actual measured gas content is far lower than the theoretically calculated gas content for the transitional shale gas reservoir. The main mechanisms are summarized to be (1) the high hydrocarbon expulsion efficiency of the “sandwich” space structure of the sandstone–shale–coal association, which gives rise to most natural gas migrating into nearby sandstone, and (2) high water saturation resulting in insufficient storage space for free gas in the shale reservoir. Unlike marine shale gas, natural gas in the transitional shale reservoir is primarily dominated by adsorbed gas in kerogen, and free gas is relatively low. The favorable lithofacies types are organic-rich siliceous/calcareous shales. Multiple layers of siderite-bearing shales/siderites are developed vertically and continuously distributed horizontally in transitional strata, particularly in flat-lagoon facies. It is easy to form a “micro-trap” to store gas in siderite-bearing shale, and siderite-bearing shale has strong sealing properties due to low porosity, low permeability and high breakthrough pressure. This property can form overpressure and trap shale gas inside the shale, providing a new research perspective for the optimization of vertical favorable intervals, as well as exploration breakthrough in transitional shale gas. Further research should strengthen the systematic sedimentological study of transitional facies, reveal shale gas occurrence state and dynamic transformation, and optimize favorable interval evaluation systems to clarify the feasibility of coal-measure gas commingled production.

Depositional Environment and Lithofacies Analyses of Eocene Lacustrine Shale in the Bohai Bay Basin: Insights from Mineralogy and Elemental Geochemistry

AbstractThe effect of various depositional parameters including paleoclimate, paleosalinity and provenance, on the depositional mechanism of lacustrine shale is very important in reconstructing the depositional environment. The classification of shale lithofacies and the interpretation of shale depositional environment are key features used in shale oil and gas exploration and development activity. The lower 3rd member of the Eocene Shahejie Formation (Es3x shale) was selected for this study, as one of the main prospective intervals for shale oil exploration and development in the intracratonic Bohai Bay Basin. Mineralogically, it is composed of quartz (avg. 9.6%), calcite (avg. 58.5%), dolomite (avg. 7%), pyrite (avg. 3.3%) and clay minerals (avg. 20%). An advanced methodology (thin‐section petrography, total organic carbon and total organic sulfur contents analysis, X‐ray diffraction (XRD), X‐ray fluorescence (XRF), field‐emission scanning electron microscopy (FE‐SEM)) was adopted to establish shale lithofacies and to interpret the depositional environment in the lacustrine basin. Six different types of lithofacies were recognized, based on mineral composition, total organic carbon (TOC) content and sedimentary structures. Various inorganic geochemical proxies (Rb/Sr, Ca/(Ca + Fe), Ti/Al, Al/Ca, Al/Ti, Zr/Rb) have been used to interpret and screen variations in depositional environmental parameters during the deposition of the Es3x shale. The experimental results indicate that the environment during the deposition of the Es3x shale was warm and humid with heightened salinities, moderate to limited detrital input, higher paleohydrodynamic settings and strong oxygen deficient (reducing) conditions. A comprehensive depositional model of the lacustrine shale was developed. The interpretations deduced from this research work are expected to not only expand the knowledge of shale lithofacies classification for lacustrine fine‐grained rocks, but can also offer a theoretical foundation for lacustrine shale oil exploration and development.

Stratigraphic Analysis and Depositional Environment of the Newly Recorded Umm Er Rhadhuma Formation (Paleocene) from the Borehole K.H12/7, South Anah City, Western Iraq

The Paleocene benthic foraminiferal zonation of the Umm Er Rhadhuma Formation from the borehole (K.H 12/7), South Anah City (Western Iraq), has been re-studied and re-analyzed precisely based on the large benthic foraminifera (LBF). They are represented by two biozone Rotorbinella hensoni Partial Range Zone, recorded from the Lower and middle parts of the Umm Er Rhadhuma Formation and Lockhartia praehaimei Partial Range Zone determined Uppermost of this unit, and dated to be the Selandian – Thanetian stage. Almost all the biogenic (micro and macro) and non-biogenic constituents, including large benthic foraminifera, Algae, Echinoderm, Bryozoans, Oyster, Gastropod fragments, and peloids, in addition to lithofacies types, indicate that this succession belongs to the Umm Er Rhadhuma Formation. Furthermore, the Paleocene shallowing upwards succession is recognized from seven identified microfacies (MF1 to MF7), which suggests three significant facies associations. A broad inner ramp represents them and is warm shallow open normal marine water (FA1). In contrast, the second facies association represents by the predominated bioclastic sand shoal facies association (FA2) and finally reaches the semi-restricted lagoon facies associations (FA3). The interaction between the local tectonic disturbance along Rutba high and eustatic sea level mainly controls the development of two sequence boundaries of Type-1 (SB1) that occurred respectively at the Cretaceous /Palaeogene K- Pg boundary and Paleocene /Eocene boundary. The Paleocene depositional system starts with major transgression during the Selandian above a sequence boundary of type one (SB.1), that separates the Late Cretaceous (Maastrichtian) successions of the Tayarat Formation from the overlying Paleocene succession with a significant gap, covering the whole Danian age (That is the top of Tectonic Megasequence AP. 9). The predominance of retrogradation staking tract indicated the transgressive system tract during the late Selandian and early Thanetian as a result of an increase in the sea level rise and expanded accommodation space. The highstand system tracts show aggradational and then change to a progradational stacking pattern by the end of the Thanetian and mark significant sea level drawdown with a new sequence boundary of type one between them.

Литофациальная интерпретация сеноманских отложений на севере западно-сибирского нефтегазоносного бассейна

The lithofacies of the Cenomanian succession within a gas field in the north of the West Siberian basin were studied using well log information especially spontaneous potential log to determine the lithostratigraphy, hydrodynamic characteristics, possible reservoir sands and depositional environment of the study area for opportunities that will support the exploration program. Geophysical well logs from six profile wells were used for this study. The lithostratigraphic model and sequence-stratigraphic frame were developed using spontaneous potential log curves to identify lithic units like sand, silt and mud. The depositional environments penetrated by the wells interpreted from log signatures gave four main types of lithofacies: Foreshore, Regressive Shoreface; Transgressive Shoreface; Discontinuous Currents were recognized from the stacking patterns of the spontaneous potential curves by modified electro facies classification. The stratigraphic column of the wells was subdivided into parasequences composing four sequences within the regional stratigraphic trend.

Sedimentary environment controls on the lacustrine shale lithofacies: A case study from the Nanpu depression, Bohai Bay Basin

Organic-rich shale of the Paleogene Shahejie Formation in the Nanpu Sag Bohai Bay Basin was analyzed using petrographic and elements analysis. These analyses were used to identify the lithofacies, reconstruct the sedimentary paleoenvironment and their relation. This paper adopted the division scheme and naming method of “Total organic carbon (TOC) + sedimentary structure + mineral composition” to carry out lithofacies identification and division. The petrographical characteristics indicated that there are 15 types of lithofacies. According to the lithologic changes, we divided the study interval from the fourth sub-member of the third member of the Shahejie formation into three parts from the bottom up. The lower part has an evident fluctuation in paleoclimate, salinity, productivity, and redox condition. The middle part has cold and arid climate conditions, the weakest weathering, the highest paleo-productivity, and anoxic conditions. The upper part was deposited under humid and warm climate conditions, strongest weathering, brackish-fresh water, lowest paleo-productivity, and dominant oxic condition. Paleo-productivity and redox conditions are the basic conditions for the enrichment and preservation of organic matter, the paleo-climate and paleo-salinity were the critical factors controlling the mineral composition and laminae development. Under warm and humid climate conditions, the shallow lake mainly develops organic-poor massive felsic shale and siltstone; the semi-deep lake is mainly developed with organic-medium laminated/layered felsic shale; the deep lake mainly develops organic-rich laminated/layered clayey shale and mixed shale. Under dry and cold climate conditions, the shallow lake area mainly develops organic-poor felsic shale, and the semi-deep lake mainly develops organic-rich layered carbonate shale and organic-rich massive mixed shale; the deep lake mainly develops organic-rich laminated/layered clayey shale.

Development model and identification of evaluation technology for Wufeng Formation–Longmaxi Formation quality shale gas reservoirs in the Sichuan Basin

In 2012, China's first national shale gas demonstrations areas were set up in the Sichuan Basin. After 10 years' construction and practice, the giant marine shale gas area of 10 × 1012 m3 level is built up in the Sichuan Basin, and shale gas steps into a new stage of large-scale benefit exploration and development. In order to systematically summarize the achievements in shale gas exploration and development and provide guidance and reference for the exploration and development of deep shale gas and shale oil & gas in other areas, this paper systematically summarizes the main characteristics, development models and key identification and evaluation technologies for quality reservoirs of Wufeng Formation–Longmaxi Formation shale gas by analyzing the electric property, lithofacies, reservoir parameters and microscopic porosity of key wells in the basin. And the following research results are obtained. First, biogenetic siliceous shale, calcareous shale and mixed shale are main lithofacies types in quality shale gas reservoirs, and they are formed in the environment of semi-deep and deep water continental shelf. Their lateral distribution is controlled by the paleogeomorphology and their vertical development is influenced by provenance, redox condition and paleo productivity. They are 25–90 m thick. Second, in the quality shale gas reservoirs develop organic pores, inorganic pore and microfractures (including lamina/bedding fractures), among which, organic pore is one of the main reservoir spaces and microfracture is not only indispensable reservoir space, but also production pathway. The reservoir space of shale gas is overall micro-nano pore, and macropores play an important role in shale gas enrichment. Third, three development models of quality reservoir are established, including sedimentary type, diagenesis type and reworking type. The sedimentary type is the foundation. Multiple quality reservoirs are developed in the high U/Th interval of graptolite belt at the bottom of Longmaxi Formation, and their thickness is mainly controlled by paleogeomorphology and especially greater in the depression area. The diagenesis type is divided into three forms, i.e., syngenetic-early diagenetic rigid support, middle-late diagenetic mineral-organic matter transformation, and overpressure relief compaction. The reworking type is dominated by quality reservoirs with microfractures. Fourth, the core technologies for identifying and evaluating quality shale gas reservoir include large-size core and rock slice observation, high-accuracy rock mineral identification, experimental gas content test and simulation, SEM microscopic characterization, 3D microscopic pore reconstruction, comprehensive geophysical interpretation and prediction and big data analysis. In conclusion, nearly 10 years' research and practice achievements in demonstration area construction can deepen the understanding on domestic quality shale gas reservoirs, promote the effective development of the theories and technologies related to shale gas reservoirs, improve the prediction accuracy of shale gas sweet spot zones/intervals, and expand the shale gas exploration and development achievements of demonstration areas.

Development and controlling factors of shale lithofacies cycles in a continental rift basin: A case study of Es4u in the Boxing Subsag of Dongying Sag, Bohai Bay Basin, China

The shale of the upper Es4 formation (Es4u), deposited during the Eocene in the Boxing Subsag of the Dongying Sag, is a typical set of lime-rich lacustrine shale in a continental rift basin. Through logging data interpretation, core and thin section observations, and geochemical elements [obtained by X radial fluorescence (XRF) mud logging] analysis, the development and controlling factors of lithofacies cycles of Es4u shale were analyzed. The results show that 22 types of lithofacies with typical characteristics are developed in Es4u, indicating the sedimentary environments, such as lower lakeshore (LL) slope zone; upper, middle, and lower shallow-lake slope zones; semi-deep-lake zone; and deep-lake zone. Lithofacies cycles in three scales are recognized in Es4u: small-scale lithofacies cycles indicated by alternate development of two lithofacies, mesoscale lithofacies cycles indicated by the repetition of lithofacies associations or regular variation of multiple lithofacies, and large-scale lithofacies cycles indicated by regular variation of well logging curves, which have the average thickness of approximately 0.4, 5, and 35 m, respectively. According to the vertical variation of astronomical parameters and lithofacies cycles, the periodic change in long eccentricity has a close relationship with the large-scale lithofacies cycle, the periodic change in slope is closely relative to the mesoscale lithofacies cycle, and the periodic change in precession is closely related to small-scale lithofacies cycle. However, the periodic change in short eccentricity correlates poorly with lithofacies cycles. The climate change reflected by long eccentricity and slope is quite distinct from the actual basin climate reflected by the paleoclimatic parameters (e.g., Rb/Sr and Sr/Cu), indicating that the basin paleoclimate was greatly influenced by the paleogeological conditions. For shale in Es4u of the Boxing Subsag, approximately 55% of the shale stratigraphic record is dominantly controlled by the astronomical climate and 45% by the paleogeological conditions. Affected by the paleogeological conditions, large-scale and mesoscale lithofacies cycles are different in lithofacies types and superimposition patterns, and just small-scale lithofacies cycles are found repetitive vertically. These findings suggest that the lithofacies cycles of shale in a continental rift basin are a product of astronomical driving mechanism (Milankovitch forcing) and variation of local paleogeological conditions, which is apparently different from the sedimentary record of marine shale that is evidently controlled by the Milankovitch cycle. Therefore, the lithofacies cycles should be comprehensively analyzed under the background of actual basin paleoclimate.

Characteristics and Influencing Factors of Multi-Scale Pore Structure Heterogeneity of Lacustrine Shale in the Gaoyou Sag, Eastern China

The success of shale oil exploration and production is highly dependent on the heterogeneous nature of the reservoir pore structure. Despite this, there remains limited research on the heterogeneity characteristics of pores at different scales in lacustrine shale oil reservoirs and the factors that impact them. This study aims to quantitatively characterize the multi-scale pore heterogeneity differences of the lacustrine shale found in the Funing Formation in Gaoyou Sag. Additionally, the study seeks to clarify the impact of the total organic carbon (TOC) and lithofacies type on pore structure heterogeneity. To achieve this, nitrogen adsorption, scanning electronic microscope (SEM), mercury intrusion porosimetry (MIP), and other experimental means were adopted in combination with the fractal dimension model of FHH and capillary. The results show that the predominant lithofacies of the Funing Formation shale samples are mixed shale (MS) and siliceous shale (SS), with a limited presence of calcareous shale (CS). The micro-pores of lacustrine shale are dominated by inorganic mineral pores and fewer organic pores. Intragranular pores and clay mineral pores are two types of inorganic mineral pores that are widely found. Small pores (pore diameter < 50 nm) make up 89% of the pore volume (PV) and 99% of the specific surface area (SSA). The fractal dimensions D1, D2, and D3 were calculated to characterize the roughness of the pore surface, the structural complexity of small pores, and the structural complexity of large pores (pore diameter > 50 nm), respectively. The increase in the total organic carbon (TOC) resulted in a decrease in the D1, D2, PV, and SSA, while connectivity showed a slight improvement. The fractal dimension of shale across all lithofacies followed the pattern: D3 > D2 > D1. The pore structure is more complex than the pore surface, and the large pores showed a greater heterogeneity than the small pores. Among the three lithofacies, CS had the largest PV, SSA, D1, and D2, indicating the development of a more complex pore structure network. This expands the space required for shale oil occurrence. However, the connectivity of the CS lithofacies is the lowest among the three, which hinders shale oil production. Although the PV of SS is slightly lower than that of CS, its average pore diameter (AVE PD) and connectivity are significantly advantageous, making SS an ideal shale reservoir. This study provides an important reference for the reservoir evaluation required to better develop lacustrine shale oil around the world.

The Characteristics of Lithofacies and Depositional Model of Fine-Grained Sedimentary Rocks in the Ordos Basin, China

In order to clarify the characteristics of fine-grained sedimentary lithofacies and the depositional models in lacustrine environments of the Chang 7 Member of the Upper Triassic Yanchang Formation in the Ordos Basin, we focus on the lacustrine lithofacies classification and controlling factors. Based on the typical field sections of the southern Ordos Basin, combined with the methods of organic geochemical analysis, polarizing microscopic observation, XRD and elemental geochemistry, we summarize the main controlling factors of the lithofacies under different sedimentary environments and establish the sedimentary model under different sedimentary backgrounds. Results show that the Chang 7 Member of the Ordos Basin includes six major lithofacies types, i.e., quasi-laminated clayey shale facies, blocky silty mudstone facies, quasi-laminated silty shale facies, laminated silty shale facies, blocky mixed mudstone facies, and laminated argillaceous siltstone facies. Al2O3, MgO, CaO, Na2O, K2O, and SiO2 are relatively depleted, while TiO2, TFe2O3, and P2O5 are relatively enriched in the Chang 72+3. Meanwhile, the trace elements are relatively enriched in Cd, Cu, Mo, U, and V. The fine-grained sedimentary facies were influenced by various sedimentary environments, such as paleoclimate, redox conditions, productivity and terrigenous input. According to lithofacies types and genetic mechanisms of the Chang 7 Member in the study area, two types of lithofacies assemblages are identified. The rapid rise in lake level during the deposition of the Chang 72+3 resulted in an anoxic water column, high productivity, and low terrigenous input under a humid climate and weak-to-moderate weathering conditions. Therefore, clayey shale lithofacies assemblage is developed in the lower Chang 7 Member.

Systematic vertical organization of matrix-rich and associated matrix-poor sandstones in ancient deep-marine slope and basin-floor deposits

AbstractDeepmarine two-part strata consisting of a sand-rich basal part overlain sharply by a mud-rich upper part have been termed linked debrites, hybrid event beds, transitional flow deposits, and bipartite facies. In continental slope and proximal basin floor strata of the Neoproterozoic Windermere Supergroup (western North America) and distal basin-floor strata of the Ordovician Cloridorme Formation (eastern North America), bipartite facies form the middle of a depositional continuum hundreds of meters long consisting upflow of thick-bedded, matrix-poor sandstone (<20% detrital mud matrix) to thin-bedded, sandy mud-stone (50%–90% mud matrix). This consistent lithofacies change is interpreted to reflect particle settling in a rapidly but systematically evolving, negligibly sheared sand-mud suspension developed along the margins (Windermere) and downflow terminus (Cloridorme) of a high-energy, mud-enriched avulsion jet.In both study areas, beds of similar lithofacies type succeed one another vertically and transform to the next facies in the depositional continuum at about the same along-strike position, forming stratal units two to nine beds thick whose grain-size distribution gradually decreases upward. This spatial and temporal regularity is interpreted to be caused by multiple surges of a single, progressively waning turbidity current, with sufficient lag between successive surges for the deposition of a traction-structured sandstone overlain by mudstone cap. Furthermore, the systematic back-stepping or side-stepping recognized at the stratal unit scale is interpreted to have been driven by a combination of knickpoint migration and local topographic steering of the flows, which continued until the supply of mud from local seafloor erosion became exhausted, the main channel avulsed elsewhere, or a new stratal element developed.

Lithofacies Characteristics and Pore Controlling Factors of New Type of Permian Unconventional Reservoir in Sichuan Basin

Alongside volcanic eruptions in the middle and late Permian, the sedimentary environment and process changed, and the lithofacies characteristics varied conspicuously in the marine deposits of the Sichuan Basin (China). The tuffaceous rocks, as a new type of unconventional reservoir, provide strong evidence for marine and volcanic influences on the lithology and reservoir potential of the rocks. With experimental studies relying on field outcrops, thin sections, scanning electron microscopy and whole-rock X-ray diffraction (XRD), the researchers analyzed the lithofacies characteristics, pore types and controlling factors on the various types of pores in the tuffaceous rocks. We identified three lithofacies types in this new type of Permian reservoir in the Sichuan Basin, namely tuff, sedimentary tuff, and tuffaceous mudstone. The mineral composition of the three lithofacies includes quartz, feldspar, carbonate minerals, pyrite, and clay, among which feldspar is mainly potassium feldspar. Tuff has high tuff content, and the lowest clay and TOC content; tuffaceous mudstones have the highest clay and TOC content, and the lowest tuff content. The pore types of the tuffaceous rocks are mainly nano-scale shrinkage pores, with a small number of intergranular pores including intragranular pores, intergranular pores, and organic pores. The shrinkage pores account for 81.9% of the total pores, and organic pores account for 11.2% of the total pores. In the tuffaceous rocks, the tuff content, quartz and feldspar content, and pyrite content are inversely correlated with porosity, while the clay content and TOC content are positively correlated with porosity. The porosity of tuff is the lowest, followed by sedimentary tuff, and the porosity of tuffaceous mudstone is the highest. Tuffaceous rocks form many micropores in the process of devitrification. Organic matter pyrolysis and organic acid dissolution also increase the reservoir space and porosity of the reservoir. This new type of reservoir has the ability of hydrocarbon accumulation along with the reservoir performance, and thus it has greater exploration prospects.

Restoration of sedimentary environment and geochemical features of deep marine Longmaxi shale and its significance for shale gas: A case study of the Dingshan area in the Sichuan Basin, South China

With the urgent need to increase shale gas production and reserves, deep shale gas has gradually become an important target for further exploration in China. However, there is still a lack in systematic research on the quality of different lithofacies types of shales in deep shale gas reservoirs. The overmature shales have a common problem, that is, the organic matter (OM) quality could not be clearly characterized by using conventional organic geochemical indicators, hindering the process of fine shale gas exploration. In this study, the deep marine Longmaxi (LMX) shale samples in the Dingshan area, southeastern Sichuan Basin were selected as a study case to investigate their lithofacies types, sedimentary environment and organic matter accumulation (OMA) mechanism, and restore the original OM attributes including original organic carbon content (TOCo) and original hydrogen index (HIo) for the different shale types to identify the favorable lithofacies for shale gas enrichment. The results show that the LMX shale can be divided into five different lithofacies, such as the OM-rich siliceous (RS) shale, OM-rich mixed (RM) shale, OM-lean siliceous (LS) shale, OM-lean mixed (LM) shale and OM-lean argillaceous (LA) shale, with a gradually decrease of their TOC contents. The TOC content of the LMX shale has a positive synergy with the indicators of paleoproductivity and redox conditions. However, since the studied area is close to the underwater highland with a high oxygen content in the bottom water and a great input of terrigenous debris, the OMA mainly depends on the level of high paleoproductivity. The analysis of sedimentary environment indicates that the OM amount and quality in various lithofacies shales are different. The RS shale at the lower part of the LMX shale has a better OM quality and a greater TOCo, which result in the greater effective organic carbon (TOCe), gas production potential, and retained gas content. It is followed by the RM, LS, and LM shales. In contrast, the LA shale lacks material base for shale gas enrichment. Combined with the available data from other deep blocks in the southern Sichuan Basin, the original OM attributes of RS and RM shales are always better than those of LM, LS, and LA shales. For deep and ultra-deep Longmaxi shale gas exploration, the RS shale should be the first choice, and then the RM shale.

Comparing “apples” and “oranges” in the Woodford Shale: Pitfalls of the thermal maturity gradient approach for constraining evolution of mudrock porosity

The thermal maturity gradient approach is widely used in comparative studies of mudrocks to assess the evolution of the pore system. The fundamental assumption in this approach is that the compared samples share similar geochemical, mineralogical and textural properties, so that any differences between them can be reasonably attributed to the impact of thermal maturity and burial alteration. However, achieving the desired range of thermal maturity often requires sampling from widely spaced localities which may result in unrecognised microtextural differences owing to regional variations in depositional controls, making it challenging to isolate the effects of thermal alteration on pore system evolution. We use samples from the widely studied siliceous Woodford Shale to highlight the importance of sample screening in characterization studies of pore evolution with progressive thermal maturity. Samples were collected from different cores spanning a gradient of thermal maturity (0.5–1.65 %VRc) across the Cherokee Platform, Arkoma and Anadarko basins, and capturing the wider geographic extent of the formation across Oklahoma. Samples were selected from the same lithofacies type but varying stratigraphic position and screened using the standard bulk geochemical and mineralogical criteria that are widely employed to ensure properties are comparable across a sample set. Notwithstanding this screening step, microscale electron microscope characterization revealed substantial microtextural and compositional differences within the sample set. Our results show that total pore volume is significantly influenced by these seemingly subtle differences, likely because pore evolution pathways vary with the abundance and distribution of primary (i.e. biogenic vs detrital grains) and diagenetic sediment constituents. We conclude that standard sample screening approaches that are limited to bulk mineralogical and geochemical characterization, and cannot identify differences in detrital, biogenic and authigenic components or account for textural differences, are insufficient for thermal gradient studies. A combination of high-resolution imaging, mineral mapping, x-ray diffraction, and low-pressure gas adsorption experiments proved better suited for sample screening ensuring an “apples” to “apples” comparison, as well as for detailed characterization bringing to light the impact of microtextural heterogeneity between samples on measured rock properties.

Effect of CO2-Brine-Rock Interactions on the Pore Structure of the Tight Sandstone during CO2 Flooding: A Case Study of Chang 7 Member of the Triassic Yanchang Formation in the Ordos Basin, China.

CO2 flooding is an effective technique to enhance oil recovery from tight sandstone reservoirs. The CO2-rich fluid reacts with the in situ minerals and results in the corrosion and precipitation of minerals in the sandstone, affecting the connectivity and morphological features of pores. However, the diagenesis of the tight sandstone is complex, and the structural modification behavior and mechanism in different lithofacies of tight sandstone during CO2 flooding are still unclear. This study combined CO2 flooding, thin-section casting, scanning electron microscopy, nuclear magnetic resonance, X-ray diffraction, and fractal analysis methods to investigate the changes in the pore structure of tight sandstone after CO2 flooding. The results show that the cement of the tight sandstone is complex and diverse. The tight sandstone can be divided into two types of lithofacies: clay cementation (CL) and ferrocalcite cementation (CA). The mineralogical alterations occur differently in each lithofacies of tight sandstone. Alterations in the cement minerals affect the pores morphology of tight sandstone depending on their mineralogical structure and texture, and the CO2 flooding mainly changes the micromorphology and heterogeneity of large pores. Clay minerals dominate the cement in the CL lithofacies of tight sandstone. The dissolution mainly occurs in small pores because the precipitation of new minerals and exfoliation of skeleton particles partially block large pores and transform them into small pores. Thus, the number of small pores of CL lithofacies increases while the number of large pores decreases. Such a phenomenon hinders the increase in porosity and permeability. On the other side, in the CA lithofacies of tight sandstone, the cement is dominated by ferrocalcite, and the dissolution of ferrocalcite is the primary mechanism of mineralogical alteration. As the ferrocalcite dissolution expands, it creates new flow paths, improving the connectivity of pores. The petrophysical properties of CA lithofacies improve significantly after CO2 flooding. There is a crucial need to study the changes in diagenetic characteristics of tight sandstone due to CO2 flooding. Such type of study provides insights related to the improvement and evaluation of the development of tight sandstone reservoirs during CO2 flooding.

Lithofacies, mineralogy, and pore characteristics of Permian marine tuffaceous rocks in the Sichuan Basin

Alongside volcanic eruptions in the middle and late Permian, the sedimentary environment and process changed, and the lithofacies and mineralogical characteristics varied conspicuously from the marine sediments in this period. Marine tuffaceous rocks beared strong witness to the marine and volcanic actions in this time. With experimental studies relying on field outcrop, thin section, scanning electron microscope, X-ray diffraction (XRD), mercury injection capillary pressure (MICP) and CT scan, the researchers analyzed the lithology, mineralogy, and pore characteristics of marine tuffaceous rocks. Among the Permian marine tuffaceous sections of the Sichuan Basin, three types of lithofacies were identified, namely tuff, sedimentary tuff, and tuffaceous mudstone. The mineral composition of the tuffaceous section includes quartz, feldspar, carbonate minerals, pyrite, clay,etc.The quartz content varies from 4.0% to 27.3%, with an average value of 13.0%; the feldspar content varies from 0 to 21.2%, with an average value of 9.8%; the carbonate mineral content varies from 8.52% to 53.45%, with an average value of 27.6%; the clay mineral content varies from 0 to 75.3%, with an average value of 44.8%; and the pyrite content varies from 0 to 13.4%, with an average value of 5.8%. The porosity of tuffaceous rocks varies from 2.2% to 8.1%, mostly concentrated in the range from 3% to 7% with an average level of 5.24%. There are mainly shrinkage pores, dissolution pores, intercrystalline pores, and organic pores. In terms of scale, the pores can be classified as micron-scale and nano-scale pores, and in terms of size, they are mainly micropores and mesopores, accounting for up to 92.12%. The pores are concentrated in the tuffaceous section and well interconnected, forming a complex organic-inorganic pore-fracture network system and bedding fractures with even better connectivity. The pores of the tuffaceous section are greatly influenced by lithofacies and mineral composition. The porosities of tuffaceous mudstone, sedimentary tuff and tuff rank downward, with average porosities of 6.5%, 5.09%, and 3.86% respectively. The felsic content is inversely correlated with porosity; the clay content and TOC content are positively correlated with porosity; the pyrite content is also inversely correlated with porosity. The marine tuffaceous section is similar to shale to a certain extent as it has relatively dense lithology, its pores are mainly of micron-scale and nano-scale and mainly include micropores and mesopores. It boasts the hydrocarbon-generating capacity and reservoir performance, serving as both a source rock and a reservoir. As a novel reservoir, the tuffaceous section can form a tight reservoir both generating and depositing gas and featuring source-reservoir paragenesis, lithological reservoir-controlling, and large-area stratified distribution, manifesting a promising future for exploration.

Identification of tight sandstone reservoir lithofacies based on CNN image recognition technology: A case study of Fuyu reservoir of Sanzhao Sag in Songliao Basin

Currently, lithofacies are automatically classified in petroleum reservoirs based on different algorithm models using well-logging vector datasets. However, well-logging vector datasets are challenging for old oil fields where there are only well-logging images dataset present. Nevertheless, tight sandstone reservoirs, which have been neglected, are becoming exploration targets in old oil fields. This has made it necessary to redefine lithofacies characteristics. Thus, this paper describes a novel, widely applicable method for using a convolutional neural network (CNN) to identify tight sandstone reservoir lithofacies, which can handle well-logging image datasets rather than well-logging vector datasets. We applied this method to the Sanzhao Sag in the Northern Songliao Basin of China using the following steps: (1) dividing typical lithofacies based on core and well-logging characteristics; (2) preparing different lithofacies’ well-logging image datasets; and (3) establishing a CNN model with training, testing, and verification. For lithofacies recognition in a tight sandstone reservoir, compared with the recognized result of a model based on a well-logging vector datasets, such as decision tree analysis (DTA; 83%), support vector machine (SVM; 81%), K-nearest neighbor (KNN; 86%), and random forest (RF; 92%), the CNN model has a better prediction accuracy (92%). It also performs well when recognizing new wells in the Sanzhao Sag (92.16%). This result shows that the CNN model can effectively identify tight sandstone reservoir lithofacies based on a well-logging image dataset. Additionally, the sample intervals, lithofacies, geologic characteristics, and sample numbers affecting the prediction accuracy of the CNN model were also analyzed.