Reservoir Quality Of Sandstones Research Articles

Reservoir quality evaluation of the Farewell sandstone by integrating sedimentological and well log analysis in the Kupe South Field, Taranaki Basin-New Zealand

The study used the sedimentological and well log-based petrophysical analysis to evaluate the Farewell sandstone, the reservoir formation within the Kupe South Field. The sedimentological analysis was based on the data sets from Kupe South-1 to 5 wells, comprising the grain size, permeability, porosity, the total cement concentrations, and imprints of diagenetic processes on the reservoir formation. Moreover, well log analysis was carried on the four wells namely Kupe South 1, 2, 5 and 7 wells for evaluating the parameters e.g., shale volume, total and effective porosity, water wetness and hydrocarbon saturation, which influence the reservoir quality. The results from the sedimentological analysis demonstrated that the Farewell sandstone is compositionally varying from feldspathic arenite to lithic arenite. The analysis also showed the presence of significant total porosity and permeability fluctuating between 10.2 and 26.2% and 0.43–1376 mD, respectively. The diagenetic processes revealed the presence of authigenic clay and carbonate obstructing the pore spaces along with the occurrence of well-connected secondary and hybrid pores which eventually improved the reservoir quality of the Farewell sandstone. The well log analysis showed the presence of low shale volume between 10.9 and 29%, very good total and effective porosity values ranging from 19 to 32.3% as well as from 17 to 27%, respectively. The water saturation ranged from 22.3 to 44.9% and a significant hydrocarbon saturation fluctuating from 55.1 to 77.7% was also observed. The well log analysis also indicated the existence of nine hydrocarbon-bearing zones. The integrated findings from sedimentological and well log analyses verified the Farewell sandstone as a good reservoir formation.

Pore structure and its control on reservoir quality in tight sandstones: a case study of the Chang 6 member of the Upper Triassic Yanchang Formation in the Jingbian oilfield in the Ordos Basin, China

This study applied vacuum-impregnated casting thin sections, fluorescence slices, scanning electron microscopy (SEM), pressure-controlled mercury porosimetry (PCP), rate-controlled mercury porosimetry (RCP), X-ray diffraction of clay minerals, overburden pressure, and conventional physical property strategies to determine the microscopic characteristics of the Chang 6 member, a typical tight sandstone reservoir in the Jingbian oilfield in the Ordos Basin, China. We also analyzed the controlling effects of pore structure on reservoir quality and oiliness. The results showed that the pore types of the Chang 6 sandstone reservoir can be divided into four categories: residual intergranular pores, dissolution pores, intercrystalline pores between clay minerals, and microfractures. The pore size of the Chang 6 sandstone reservoir ranged from 20 to 50 μm. We employed PCP and RCP strategies to characterize the pore structure of the Chang 6 reservoir. The pore radius was less than 2 μm, and on average, the throat radius was less than 0.3 μm. The reservoir physical properties were affected by diagenesis, particularly compaction, and the average porosity failure rate was 56.3%. Cementation made the reservoir more compact, dissolution improved the physical properties of the reservoir locally, and fracturing effectively improved the reservoir seepage ability; however, its influence on porosity was limited. The pore structure controlled the quality of the reservoir. The physical properties of the reservoir were closely related to the oil-bearing properties. The lower limits of porosity and permeability of industrial oil flow in the reservoir were 7.5% and 0.15 mD, respectively. These results provide an additional resource for the exploration and development of tight oil in the Ordos Basin.

Diagenetic alteration, pore-throat network, and reservoir quality of tight gas sandstone reservoirs: A case study of the upper Paleozoic sequence in the northern Tianhuan depression in the Ordos Basin, China

The Sulige gas field in the Ordos Basin is the largest gas field in China. The studied area, the northern Tianhuan depression, is located west of the gas field and is considered as a potential gas-producing area. The complex diagenetic alteration history and its effects on the pore-throat network and reservoir quality of the tight gas sandstones are explained. The upper Paleozoic first member of the Shanxi Formation and eighth member of the Shihezi Formation tight gas reservoir rocks are composed of coarse- to medium-grained sandstones and are dominated by sublitharenites. The most important diagenetic factor for porosity reduction was compaction, followed by cementation. Diagenetic mineral phases include quartz overgrowth, calcite cement and replacement, and kaolinite cement and replacement, with minor heulandite and chlorite. Pressure dissolution of quartz grains and quartzite fragments is the main source of silica for the early and late quartz overgrowth cementation (formed mainly from 120°C to 140°C). The hydrolysis and alteration of volcanic material and mica flakes released some silicon, calcium, and aluminum ions. Calcium and carbon in calcite cements could be derived from surrounding mudstones. Calcite replaced the quartz overgrowths. The formation of kaolinite, as cement and replacements, was controlled by the interactions of silica, aluminum, pH, and temperature (140°C). Because of deformation of detrital components (mica flakes and ductile rock fragments) and diagenetic replacement, clear negative correlations between reservoir quality and contents of the diagenetic minerals are lacking. A complex and isolated pore-throat network was created by strong diagenetic transformation. Based on analysis of thin sections, three-dimensional x-ray micro–computed tomography, and constant-rate mercury injection data, the samples with relatively good reservoir quality (porosity >10% and permeability >0.5 md) are characterized by dissolution pores and primary intergranular pores and commonly contain more and larger pore throats both in volume (counts >1800 and size >0.02 per unit volume) and diameter (>1.20 μm) than the samples with poor reservoir quality. The intercrystalline pores of kaolinite are very widespread and considered favorable for oil and gas accumulation because some asphalt was found within these pores. This study may be used, by analogy, to better understand diagenetic factors controlling the formation of tight gas sandstone reservoirs and predict the development of pore-throat systems.

Comparison of diagenetic characteristics and pore evolution in outcrops and cores of tight sandstone reservoirs in the Triassic Yanchang Formation, the Ordos Basin, China

The sedimentary and diagenetic control on tight sandstone reservoir quality is a key focus of hydrocarbon exploration. This paper compares reservoir characteristics, such as diagenetic processes, pore types, and porosity evolution, of cores and outcrops of the Yanchang Formation, the Ordos Basin, northwestern China, to identify diagenetic trends and processes responsible for improving reservoir quality and to develop a model for identifying reservoir sweet spots in tight sandstone formations. Samples of the Yanchang Formation sandstone are taken from wells and adjacent outcrops in the Ordos Basin. No significant differences were found in porosity-destructive diagenesis such as compaction and cementation between the core and outcrop samples. However, the effects of dissolution are far more pronounced in cores than in the outcrop, resulting in a higher core sample porosity. High-density sampling of the outcrop revealed that the reservoir sweet-spot model is controlled by sedimentation, whereas three distinct vertical diagenetic zones were identified in adjacent wells. A comprehensive analysis of cores and outcrop samples shows that both areas experienced similar sedimentation and early diagenetic patterns. However, the diagenetic processes experienced by core and outcrop sandstones diverged during the later part of burial and uplift, with dissolution being responsible for the contrasting reservoir porosities between core and outcrop.

Genetic mechanisms of Permian Upper Shihezi sandstone reservoirs with multi-stage subsidence and uplift in the Huanghua Depression, Bohai Bay Basin, East China

Braided river sandstones in the Permian Upper Shihezi (US) Formation in the Huanghua Depression, Bohai Bay Basin, East China, are reservoirs with large accumulations of hydrocarbons. The Upper Paleozoic formations, including the Carboniferous and the Permian strata experienced multi-stage of subsidence and uplift in the past 320 Ma, leading to occurrence of subaerial exposure and deep burial of the Permian US sandstones. Due to complex tectonic evolution, the Upper Paleozoic formations in different tectonic zones of the Huanghua Depression experienced various burial-thermal histories, resulting in the development of long-term open (LTO), early-open to late-closed (EOLC), and long-term closed (LTC) geochemical systems in different Permian US sandstones, as well as different evolutionary patterns of diagenesis and reservoir quality. After extensive diagenetic alteration, the present sandstones are primarily quartzose, subfeldsarenite, and sublitharenite sandstones. Pores in shaly sandstones and fine-grained sandstones were destroyed extensively, only pebbly to medium-grained sandstones can be potential effective reservoirs. The Permian US sandstones with the LTO systems, which are currently overlain by the Paleogene strata and buried less than 2500 m, experienced two stages of meteoric freshwater leaching and are characterized by extensive feldspar dissolution and weak precipitation of authigenic kaolin and quartz. Intergranular and intragranular pores dominate the 10–20% porosity, and permeability ranges from 1mD to 800 mD. The Permian US sandstones with the EOLC systems, which are currently overlain by thick Jurassic and Cretaceous strata on a regional scale and buried deeper than 3000 m, experienced both meteoric freshwater leaching and deep burial leaching and are currently characterized by extensive feldspar dissolution, extensive kaolin precipitation, and weak-moderate quartz cementation. Intragranular secondary pores and kaolin intercrystal pores dominate the 5–15% porosity, and permeability ranges from 0.1 mD to 10 mD. The Permian US sandstones with the LTC systems, which are currently overlain by a thick Permian Shiqianfeng Formation on a regional scale and buried deeper than 3000 m, experienced no meteoric freshwater leaching but only burial dissolution; they are currently characterized by extensive feldspar dissolution and extensive precipitation of clays and quartz cements. Intercrystal pores in the authigenic clay and intragranular secondary pores dominate the 5–10% porosity, and permeability ranges from 0.01mD to 1mD. Since the unconformities, overlying strata, and tectonic evolution relevant to the Permian US sandstone unit can be analyzed using seismic data, the proposed genetic models can support the predrill evaluation of reservoir quality of the Permian sandstones.

The impacts of diagenetic facies on reservoir quality in tight sandstones

Abstract The impact of diagenetic minerals and the characteristics of pore structures on reservoir qualities has been studied separately in the past years. However, the difference in the reservoir quality with different pore structures and having same or similar content of diagenesis minerals has not been ascertained. In this study, based on the core samples derived from Chang 6 member in the Ordos basin, various tests were performed to examine the sandstone diagenesis and investigate the pore structure. The results showed that there were five diagenetic facies by diagenetic and pore structure analyses, and the best reservoir quality rocks were found to have relatively low percentage of illite, carbonate cement, pore-filling chlorite, authigenic quartz, and relatively high proportion of intergranular pores. Smectite-to-illite reaction and chemical compaction were main sources for quartz cementation at 60–120°C, and carbonate content was found to increase toward source rocks. The porosity depth trends significantly affected the diagenetic facies. The diagenetic and the pore structure pathways of various diagenetic facies were reconstructed by integrated petrographic, mineralogical, and pore system data. This study provides insights into the porosity evolution and diagenetic pathways of various diagenetic facies of tight sandstones, and the influence of diagenesis minerals and pore structures on their reservoir quality.

Multiple organic–inorganic interactions and influences on heterogeneous carbonate‐cementation patterns: Example from Silurian deeply buried sandstones, central Tarim Basin, north‐western China

AbstractThis study investigates the importance of multiple organic–inorganic interactions on heterogeneous carbonate cementation patterns in Silurian deeply buried sandstones, central Tarim Basin, north‐western China, to evaluate their effects on reservoir quality. Petrographic observations and mineral geochemistry identify two stages of carbonate cementation: (i) eogenetic, poikilotopic blocky calcite precipitated at 20 to 50°C (δ13CVPDB from −0.3‰ to +6.8‰; δ18OVPDB between −6.6‰ and −2.9‰) and dolomite precipitated at 40 to 62°C (δ13CVPDB from +1.9‰ to +5.3‰; δ18OVPDB between −6.2‰ and −2.1‰); and (ii) mesogenetic, isolated pore‐filling ankerite formed at 70 to 120°C (δ13CVPDB from −14.4‰ to −6.6‰; δ18OVPDB between −11.6‰ and −7.8‰). The eogenetic carbonates are predominantly distributed as carbonate‐cemented beds or concretions along sandstone–mudstone contacts and were derived mainly from microbial methanogenesis of organic matter and dissolution of carbonate minerals in adjacent calcareous mudstones. The mesogenetic ankerite cements occur predominantly as carbonate‐cemented beds, concretions and patches, and are more concentrated in the central sections of sandstone bodies. It is inferred that ankerite cements were sourced from dissolution of eogenetic calcite and dolomite via in situ generation of organic CO2 related to thermal decarboxylation of organic acids within sandstones. Kinetic modelling results coupled with petrographic observations illustrate that mesogenetic dissolution of eogenetic calcite and dolomite cements did not enhance total reservoir porosity due to re‐precipitation of ankerite cements on a very local scale within sandstone bodies. The development of extensively carbonate‐cemented geometries (beds, concretions and patches) created by different generations of carbonate cements derived from external sources, led to reservoir heterogeneity and significant destruction of sandstone reservoir quality especially during diagenesis. The results of this study demonstrate the importance of multi‐stage, intra‐formational mass transfer induced by concentration gradients during organic–inorganic interactions in mediating a variety of spatial patterns of carbonate cementation under different diagenetic regimes. Better insights of these organic–inorganic interactions in a coupled sandstone–mudstone system could improve predictive models of heterogeneous diagenetic alterations in deeply buried sandstones of similar origin.

Sedimentological analysis and reservoir quality of the early Cenomanian transgressive sandstones, northern Western desert, Egypt

Depicting depositional models of transgressive sandstone reservoirs is challenging due to their internal complexity and limited case studies. This study presents an example for transgressive sandstone reservoirs from the Bahariya Formation at Hayat-Yasser oil field, Shushan Basin (northern Western Desert, Egypt). The upper part of the Bahariya Formation accumulated in an estuarine setting as a consequence of a successive sea level rise. Detailed reservoir characterization, employing sequence stratigraphic and sedimentological approaches, revealed that the studied reservoir consists of a mixed siliciclastic-carbonate transgressive estuarine-shelf succession. Four conventional core samples of this succession at Khalda ridge (Shushan Basin), permit the analysis of their sedimentology and internal architecture (mesoscopic heterogeneities) along correlation lines parallel or slightly oblique to depositional dip. The succession consists of six backstepping parasequence sets that record the overall shift from tidal-dominated to wave-dominated estuaries/bays. The transgressive component of the parasequences sets includes bay mudstones, flood-tidal deltas, and shoreface/barrier deposits; while the regressive component includes estuarine point bars, bay head deltas, and tidal deltas. The reservoir properties show significant variation according to the facies assemblages. The tidally-dominated estuarine channels show low net-to-gross, as it is characterized by lenticular sandstone bodies with several lateral and vertical baffles and barriers. On the other hand, the shoreface/barrier and tidal delta sandstones show high net-to-gross and consist of amalgamated sandstone bodies with little mud barriers or baffles. Petrographically, the upper Bahariya reservoir includes intergranular and secondary porosities. The microscopic heterogeneities are mainly due to the fine grain nature, poor sorting, presence of matrix, and continuous mud barriers or baffles. Destruction of porosity, affecting the reservoir quality, occurred by silica overgrowths, ferroan calcite, grain coating chlorite, and kaolinite cementation, as well as compaction. Feldspar dissolution is the main diagenetic process responsible for the formation of secondary porosity, enhancing the reservoir quality.

Effective hydrocarbon-bearing geological conditions of the Permian strata in the South Yellow Sea Basin, China: Evidence from borehole CSDP-2

For nearly 60 years, petroleum geologists have been puzzled by oil and gas exploration in the South Yellow Sea basin, which is considered to have broad oil and gas prospects, but no commercial hydrocarbon reservoirs have been obtained. The hydrocarbon-bearing geological conditions of the Permian strata with good exploration prospects also have never been systematically investigated. To solve the problem, a series of experiments was conducted on core samples obtained from borehole CSDP-2, the first exploration well with continuous coring in the South Yellow Sea Basin, including petrology analysis, geochemical testing, pore-throat testing, hydrocarbon-charging analysis, and sealing-ability evaluation. Vitrinite reflectance (Ro), total organic carbon (TOC), pyrolysis, extractable organic matter (EOM) and kerogen microscopic examination were carried out to evaluate the quality of the Permian source rocks. Porosity, permeability, thin section, scanning electronic microscopy (SEM) and rate-controlled mercury injection (RCMI) analyses were combined to verify the effectiveness of sandstone reservoirs. The sealing capacity of caprock was mainly evaluated by the displacement pressure. And according to the fluid-inclusion analysis and oil-source correlation, the hydrocarbon accumulation events were determined. After comprehensive analysis, we clarified the effective hydrocarbon-bearing geological conditions and favorable prospects in the Permian of the South Yellow Sea Basin. Furthermore, the main controlling factors pertaining to favorable source rocks and tight sandstone reservoirs were ascertained in this paper.The results show that two sets of the Lower Permian Qixia and Gufeng Formation (P1q-g) and the Upper Permian Longtan and Dalong Formation (P2l-d) source rocks, along with the P2l-d tight sandstone reservoirs, compose two types of hydrocarbon accumulation assemblages, which are effectively sealed by the Lower Triassic Qinglong Formation (T1q) roof caprock. One type represents the ‘lower-generation and upper-accumulation’ between the P1q-g source rocks and the P2l-d sandstones, and the other type represents the ‘self-generation and self-accumulation’ in the Upper Permian strata. Vertical stacking between the Permian source rocks and sandstone reservoirs ensures the near-source hydrocarbon accumulation, and faults and microcracks can be regarded as beneficial channels for hydrocarbon migration. In addition, we find that the quality of source rocks is improved by large-scale marine transgression, high bio-productivity, and tectonic–thermal events. The quality of tight sandstone reservoirs is more dependent on throats with relatively larger radius than pores. This study provides important enlightenment for exploring marine hydrocarbons in the South Yellow Sea Basin, and can be useful in understanding the dominant geological factors affecting favorable source rocks and sandstone reservoirs.

Origin of multiple-phase carbonate cements in the sandstones of the third member of the Shahejie Formation in the Niuzhuang Sag, Bohai Bay Basin

Authigenic carbonate cement is one of the most abundant diagenetic minerals in sandstone reservoirs. Determining its origin and distribution may provide useful information for understanding the sandstone reservoir quality. In this study, we report results from a suite of analytical techniques to investigate the origin and evolution of carbonate cements in the third member of the Shahejie Formation in the Bohai Bay Basin. Our data indicate that the carbonate cements mainly occur in three phases: the early-phase calcite, late-phase ferroan calcite, and late-phase ferroan dolomite and/or ankerite. The early-phase calcites show depleted δ18O of the early-phase calcite (−11.8% to −7.8%), suggesting an 18O-depleted fluid origin from the mixing between lacustrine and meteoric waters. They were precipitated earlier than the quartz overgrowth at 40°C–63°C based on the oxygen isotope. The late-phase calcites were precipitated at 70°C–115°C, and they originated from water-rock interaction modified pore water at the same time or later than feldspar leaching. They show a lower average δ13C value (1.27%) than the early-phase calcite (1.65%), indicating that the interbedded shales within the sandstones most likely provided the required components for the precipitation of the late-phase calcite. Also, some Fe2+ was released during the organic acid release and then precipitated the late-phase ferroan calcites. The late-phase ferroan dolomite and/or ankerite were precipitated at 90°C–137°C in a deep diagenetic condition. They show depleted δ13C values (mean [Formula: see text]), and the carbons were mainly sourced from the thermal decarboxylation of organic matter and lacustrine carbonate. The early-phase calcite inhibits compaction while filling the pores, and the dissolution of ferroan calcite cements was the main reason for the development of secondary pores in the sandstone reservoirs. The late-phase ankerite reduces the reservoir porosity.

Formation of abnormally high porosity/permeability in deltaic sandstones (Oligocene), Bozhong depression, offshore Bohai Bay Basin, China

The porosity-enhancing capability of secondary porosity in a closed sandstone geochemical system has been suggested to be small in past studies. However, the mechanisms responsible for the formation of abnormally high porosity/permeability remain unclear. The Oligocene Dongying Formation in the Bozhong Depression, offshore Bohai Bay Basin, preserves abnormally high porosity, i.e., 36.4% (average) or 42.1% (median) higher than background values. This work selects samples with abnormally high porosity (AHP), samples with normal porosity (NP) and samples with abnormally low porosity (ALP) and compares their differences in physical properties (porosities, permeabilities and thin section porosities), diagenetic minerals, pore structures, compaction and cementation, detrital compositions and grain texture in an attempt to find the real factors controlling this heterogeneity. An effective export of dissolution by-products (Al3+ and Si4+) from AHP to NP and ALP leads to 2.01% (average) or 2.11% (median) newly created porosity in AHP, which is still significantly lower than the porosity difference between AHP and NP. In comparison, compaction reduction and mechanisms beneficial for compaction reduction should be the major factors controlling AHP formation. The content and distribution pattern of early carbonate cements, compared to the sedimentary factors (grain size and rigid grains), have significant influences on compaction reduction. Only samples with moderate contents (0.5–3.0%) of uniformly point-distributed early carbonate cements and with effective export of dissolution by-products can form AHP. Three formation models for ALP and two formation models for NP are also proposed. In summary, the evolution of porosity/permeability is controlled by early carbonate cements and induced compaction reduction (major causes) and by feldspar dissolution and redistribution of by-products (minor causes). This work provides new constraints on the formation of AHP and heterogeneity and is therefore important for sandstone reservoir quality prediction.

Diagenesis and reservoir quality of Paleocene tight sandstones, Lishui Sag, East China Sea Shelf Basin

The Lishui Sag in the East China Sea Shelf Basin (ECSSB) has significant potential hydrocarbon resources, with Paleocene sandstones as its main oil and gas-bearing reservoir. The sandstones in the Paleocene formations are mainly composed of medium-grained, moderately sorted litharenite. In this study, distributions of the porosity and permeability reveal that the Paleocene sandstones have the characteristics of medium porosity and low permeability. The sedimentary microfacies, primary texture, diagenesis, and bioturbation jointly control the reservoir quality. Sandstones with distributary channel and sand bar sedimentary microfacies as well as better sorting tend to exhibit a higher quality. Compaction and carbonate cementation are the major determinants to reductions in porosity and permeability. Grain-coating chlorites within a certain content range may play a positive role in the preservation of porosity in eodiagenesis. Early dissolution has a constructive effect on the reservoir quality. In contrast, the dissolution products such as kaolinite and dawsonite cannot move over a long distance and precipitate nearby to block the intergranular pores when reservoir closure increases. Therefore, dissolution in a relatively closed geochemical reservoir system may contribute little to porosity. Contrastingly, the permeability of the Paleocene reservoir may be reduced by the conversion of the pore network. In the Lishui Sag, the bioturbation plays a role in the deterioration of reservoir quality in a limited scale by decreasing reservoir sorting and increasing compaction.

Controls of diagenetic alteration on the reservoir quality of tight sandstone reservoirs in the Triassic Yanchang formation of the Ordos Basin, China

The Ordos Basin is rich in tight oil resources, and the Upper Triassic Yanchang Formation tight oil reservoir is one of the most important exploration targets. In the study area, the Chang 63 tight sandstone reservoir exhibits lacustrine deep-water lobe-shaped delta deposits with low porosity, low permeability and a complex diagenetic alteration history. The reservoir quality is extremely heterogeneous because of the heterogeneity of the depositional lithofacies and diagenetic alterations. This research integrates multiple techniques to investigate the depositional lithofacies, the characteristics and distributions of various diagenetic alterations, and their controls on reservoir quality. Four lithofacies are identified, and the various lithofacies have different diagenetic characteristics and diverse reservoir qualities. The Chang 63 tight sandstone reservoir has experienced significant diagenetic alteration. Mechanical compaction and authigenic illite are the most important causes of the decrease in reservoir quality. Carbonate cementation preferentially occurs in sandstones of lithofacies “i“ and “ii” near the mudstone-sandstone contact surfaces. Sandstones within 1 m of adjacent mudstone-sandstone contact surfaces are usually tightly cemented by carbonate cement. Illite can be divided into two types according to its morphology. Both types of illite lead to a decrease in reservoir quality. Ultimately, by combining the relationship between depositional facies and lithofacies with the distribution of different diagenetic processes, five evolutionary patterns are identified for reservoir quality in the lacustrine deep-water lobe-shaped delta deposits can be summarized by ratings of best, moderate, relatively poor, poor and worst.

Impacts of Pore-Throat System on Fractal Characterization of Tight Sandstones

The pore-throat structures play a dominant role in the evaluation of properties of tight sandstone, but it remains difficult to determine the related parameters and understand their impact on reservoir quality. Hence, toward this end, we analyze the experimental data that are indicative of the pore-throat system, then we investigate the effect of fractal dimensions of pore-throat structures on petrologic and physical properties, and finally, the optical observations, fractal theory, and prediction model were integrated to explore the qualities of various reservoir types in tight sandstones. The results show that the fractal dimensions of the mercury intrusion curve correspond to three pore-throat types and those of the mercury extrusion curve could correspond to two pore-throat types. Five types of reservoirs were identified, the best reservoir type has a high percentage of interparticle and dissolution pores but a low proportion of clay-related pores, and the differences in pore-throat connectivity of various types affect storage capacity significantly. The storage ability prediction models of various reservoir types are raised by integrated experimental data. This work employed a comprehensive fractal theory based on capillary pressure curves and helps to explore how pore-throat systems influence reservoir quality in tight sandstones.

Linking the influence of diagenetic properties and clay texture on reservoir quality in sandstones from NW Borneo

Reservoir studies in the Sandakan sub-basin NW Borneo are challenging and inexplicable due to complex diagenetic effects such as compaction, cementation, dissolution and depositional means including grain size, sorting and packing with reference to clay texture. These diagenetic effects together impart significant amounts of uncertainty on reservoir quality assessment. In this regard, we documented the mineralogical, diagenetic and reservoir properties of sandstones from the Sandakan Formation, NW Borneo through thin sections, SEM, XRD, XRF, He-porosimeter and poroperm. In order to analyze the influential parameters, we used matrix, grain rearrangement and clay coating around detrital grains for the reservoir quality. Our results show that the sub-litharenites to quartz arenite that dominate in the sandstones are drawn from recycled orogenic sand. Furthermore, reservoir properties of the sandstones were ascertained to be controlled by diagenesis, micro quartz with clay coatings and pressure dissolution of feldspar with the alteration of lithic fragments. The result shows that the physical compaction, rearrangements and ductility of grains were the reasons that reduced porosity and permeability. The main reason for porosity reduction in the present study is pressure dissolution that resulted in the form of chemical compaction induced by micas with grain contacts. Nonetheless, through SEM and petrography, clay coatings around the grains and dissolution of feldspar were found to optimally contribute to the preservation of reservoir properties. Dissolution of feldspar and kaolinization resulted in the formation of secondary porosity that allows meteoric water to percolate through the open and detachment spaces of quartz or other grains. This percolation is also responsible in enhancing the pore throat radii in the reservoir media of the analyzed sandstones. These traits are obliging to diagenetic effects on porosity and permeability, which are linked with the physical compaction and clays coating to reduce and preserve the reservoir properties respectively of same type of reservoirs globally.

Diagenesis and reservoir quality of lacustrine deep-water gravity-flow sandstones in the Eocene Shahejie Formation in the Dongying sag, Jiyang depression, eastern China

The lacustrine deep-water gravity-flow sandstone reservoirs in the middle of the third member of the Shahejie Formation (43.7–38.2 Ma) are the main exploration targets for hydrocarbons in the Dongying sag, Jiyang depression, eastern China. The impact of diagenesis on reservoir quality of these sandstones has been established by a variety of methods. Petrographically, the sandstones consist mainly of lithic arkose (with 43% of quartz, 33% of feldspar, and 24% of lithic fragment) with reservoirs having average porosity and permeability values of 17.1% and 38.1 md, respectively. In addition to their primary intergranular pores, secondary pores are well developed from feldspar dissolution, with pore-throat radii ranging from 0.01 to 40 μm. The dissolution of feldspar and transformation of smectite to illite may be the sources of silica for the first phase of quartz overgrowth, whereas illitization of kaolinite may be responsible for the second phase of quartz overgrowth. In addition, pressure dissolution of quartz is also an important source of silica for quartz overgrowth. Dissolution of feldspar has little impact on reservoir quality, whereas compaction and carbonate cementation are the main factors significantly reducing reservoir quality. Oil charge may have retarded carbonate cementation and generated overpressure that retarded compaction, thus preserving porosity. Precipitation of vermicular kaolinite, resulting from feldspar dissolution, promoted a change in the wettability of the reservoir, from water wet to oil wet, which is beneficial for oil entrapment. The central parts of medium- to thick-bedded sandstones that have experienced oil charge twice appear to have the highest-quality reservoirs based on petrophysical properties. This work provides a new interpretation of the formation mechanism of high-quality lacustrine deep-water gravity-flow sandstones reservoirs that have experienced multiple periods of oil charge.

Authigenic clay minerals and calcite dissolution influence reservoir quality in tight sandstones: Insights from the central Junggar Basin, NW China

Authigenic clays and calcite cements are important in the development of reservoir tightness and the formation of hydrocarbon sweet spots. We investigated Jurassic low-permeability sandstone reservoirs in the central Junggar Basin, NW China, using petrography, mineralogy, porosity, and permeability assessment, and stable C and O isotope analysis to ascertain the influence of authigenic clays and calcites on reservoir quality. Here, we establish the properties and diagenetic processes of the reservoir sandstones, and construct a generalizable model of reservoir quality. The results show that the sandstones are mainly litharenite and feldspathic litharenite and can be classified into ductile-lithic-rich sandstones and ductile-lithic-poor sandstones according to rock composition. The ductile-lithic-rich sandstones are tight (mean porosity = 7.31%; mean permeability = 0.08 mD) as a result of intense compaction. In contrast, the ductile-lithic-poor sandstones can be classified into five types according to diagenetic process. The formation of favorable hydrocarbon reservoir properties is closely related to the presence of authigenic clays and dissolution of calcite. In particular, kaolinite fills intergranular pores, thereby blocking pore space and reducing reservoir quality. Chlorite coating resists compaction and limits the formation of quartz overgrowths, thereby preserving pore space and enhancing reservoir quality. Calcite controls reservoir quality through both precipitation and dissolution. Calcite precipitation results in reduced reservoir quality, whereby early calcites that were precipitated in formation water resist compaction and provide the basis for subsequent dissolution and late precipitation, whereas dissolution of calcite in mesodiagenesis improves reservoir quality. A generalized model is formulated by relating diagenetic facies types to depth and porosity, providing a reference for other similar reservoirs. Our data suggest that deeply buried tight sandstones can be exploration prospects under favorable conditions involving the presence of authigenic clays and dissolution of calcite, as in the central Junggar Basin of this study.

Diagenetic evolution and its impact on reservoir quality of tight sandstones: A case study of the Triassic Chang 6 Member, Ordos Basin, northwest China

The Triassic Chang 6 Member sandstones are important oil reservoirs in the Ordos Basin of north-western China, and are actually considered to be typical tight sandstone reservoirs. Diagenetic alterations play a prominent role in considerably impacting the reservoir quality. In this study, an integrated approach of: thin sections, scanning electron microscopy (SEM), X-ray diffraction (XRD), cathodoluminescence (CL), fluid inclusions, laser Raman spectrographic analysis, and stable isotope analysis was applied to examine the diagenetic history recorded in the Chang 6 tight sandstones and investigate the influences of diagenesis on reservoir quality. The sandstones consist of fine-grained, moderated-to-well sorted arkoses (Q48.5F40.9R10.6) and lithic arkoses (Q54.3F29.1R16.6) with an average porosity and permeability of 9.10% and 0.39 mD, respectively. The sandstones have undergone complex diagenetic processes; dominant eogenetic features include: (ⅰ) mechanical compaction, (ⅱ) leaching of feldspar in meteoric water, (ⅲ) chlorite coats originating from the transformation of precursor clays and precipitating directly from interstitial water, (ⅳ) calcites filling intergranular pores as poikilotopic cements, and (ⅴ) quartz and albite overgrowths on surfaces of grains. Subsequent mesogenetic events include: (ⅰ) ankerite cements filling porosity and replacing framework grains, (ⅱ) extensive dissolution of feldspars and precipitation of kaolinites due to emplacement of organic acids, and (ⅲ) secondary quartz overgrowths in the late period of mesogenesis. Early calcite is characterized by 13C (δ13CPDB ranging from −3.45‰ to −0.30‰, mean = −2.00‰) and 18O (δ18OPDB varying from −16.70‰ to −5.80‰, mean = −10.50‰), while late ankerite is rich in 12C (δ13CPDB ranging from −10.71‰ to 0.80‰, mean = −3.58‰) and 16O (δ18OPDB varying from −24.30‰ to −15.80‰, mean = −19.93‰). Moreover, calcite hosts single liquid fluid inclusions, two-phase fluid inclusions are contained in ankerite cements (with mean homogenization temperature (Th) of 96.0 °C and salinity estimates from 11.47 to 18.96 eq. wt.% NaCl). These data indicate that the calcite originated from meteoric water and the ankerite was associated with organic matter and recrystallization of the early calcite cements. Furthermore, the metallic ions such as Fe2+, Mg2+, and Ca2+ required for ankerite precipitation were produced through alterations of volcanic fragments, feldspars, and biotites. In comparison, compaction has more important role than cementation in the loss of original porosity although cementation exerts enormous influence on reducing porosity. Quantitatively, the compactional porosity loss reaches 54.9%, while the cementation porosity loss accounts for up to 42.8%. Carbonate cements and the other authigenic products (quartz and albite overgrowth, and kaolinite) mostly fill intergranular pores, thus reduce porosity. Chlorite coats have effects in inhibiting compaction and restraining the development of quartz overgrowth, thereby protect the intergranular porosity to some extent. Early secondary porosity was enhanced by leaching and dissolution of feldspar in the eogenetic period. However, these processes did not increase the net porosity in the mesogenetic stage.

Mudstone/sandstone ratio control on carbonate cementation and reservoir quality in Upper Permian Rotliegend sandstones, offshore the Netherlands

Abstract The eolian-fluvial sandstones of the Upper Permian Rotliegend formation, which were deposited in the Southern Permian Basin, are today deeply buried (~3–4 km) and constitute important gas reservoirs in the Netherlands and the southern North Sea. The reservoir properties of the sandstones have been documented to be strongly affected by diagenesis, but the primary diagenetic factors impairing reservoir quality and their cause remain variably interpreted in the literature. Here, we present the results of a detailed investigation on the diagenetic processes controlling reservoir quality in the Lower Slochteren formation in the L and K blocks offshore the Netherlands, where fluvial and aeolian sandstones intercalate with playa lake muds in a delta setting. Quantitative analysis of the diagenetic mineral phases occurring in all main depositional facies (eolian, fluvial, playa-lake) was carried out on more than 200 samples from 21 wells, with the authigenic mineral composition of an additional 500 samples being evaluated qualitatively. The integration of petrographical observations with log data and core descriptions reveals that cement distribution/abundance is not dominantly driven by depositional facies, nor are reservoir properties. Early pore-filling dolomite cement can be as high as 40% and represents the main control on reservoir properties. Detailed analysis of its spatial distribution shows it to be distinctly related to mudstone proximity and mudstone/sandstone (M/S) ratio. Sandstones occurring as thin beds in mudstone-rich depositional sequences (high M/S ratio) typically exhibit strong pervasive carbonate cement regardless of sedimentary facies. In contrast, sandstones forming thick beds in mudstone-poor sequences (low M/S ratio) are commonly free/low in dolomite cement. Our results demonstrate, for the first time, that reservoir quality in the Rotliegend sandstones in the delta setting of the Netherlands is primarily controlled by early dolomite cement. The latter is most developed in areas with high (>70%) vertical M/S ratio, where it may be a devastating factor for reservoir quality. Best reservoir sandstones should be expected where the depositional stacking pattern is poor in shaly deposits (playa-lake and distal sheet-flood sediments), such as in the southern/south-eastern part of the studied area.

Investigation of pore structure and reservoir quality of Eocene beach-bar sandstones in Dongying Depression, Bohai Bay Basin, East China

The Eocene beach-bar sandstones are among the most important petroleum exploration targets of Dongying Depression, China. A little is known about the heterogeneous pore structure characteristics, contributing factors and their relations with highly variable macroscopic petrophysical properties. In this study, various testing techniques and methods have been conducted to investigate the depositional and diagenetic features, petrophysical properties and pore structure characters. Three major lithologies, namely fine sandstones, siltstones and argillaceous siltstones, are identified with distinct sandstones texture, diagenetic components and petrophysical properties. Relatively good sorting and large particle size are the dominant sedimentological factors that result in well-preserved primary porosity of fine sandstones. The exceptionally high permeability of argillaceous siltstones with low porosity is largely related with micro-fractures. Compaction is the main porosity-reducing factor of beach-bar sandstones, in addition to cementation of carbonate, quartz overgrowth and clay. Dissolution process crucially depends on the preservation of primary porosity, and it is often limited due to strong compaction. The reconstructed pore network and MICP experiments both suggest that the pore space connectivity is closed related with pore and throat size, and large pore throats are more likely to be connected by neighboring throats and form connected paths in the sandstones. The large pore systems determine the reservoir quality by contributing the vast majority of permeability, even if they may occupy small proportion of pore volume. The pore structures of fine sandstones mainly consist of large pore systems, which cause favorable high reservoir quality and strong correlations between porosity and permeability. The unfavorable reservoir quality, great fractal dimensions and poor correlations between porosity and permeability of argillaceous siltstones samples are largely attributed to the dominating micro pore throats.