Distributary Channel Sandstones Research Articles

Impacts of sedimentary characteristics and diagenesis on reservoir quality of the 4th member of the Upper Triassic Xujiahe formation in the western Sichuan basin, southwest China

The 4th member of Xujiahe Formation (Xu4) in the western Sichuan Depression is a typical porous tight sandstone reservoir. Finding reservoirs with relatively high porosity controlled by sedimentation and diagenesis is crucial for successful tight gas exploration. In this study, the Xu4 sandstone was studied by diverse methods, including outcrop and core observation, logging data interpretation, thin section and cathodoluminescence examination, scanning electron microscopy analysis, and X-ray diffraction, to determine sedimentary and diagenetic characteristics and to discuss their impact on the sandstone reservoir quality. During the depositional period of Xu4, subaqueous distributary channel sandstone of braided river delta front developed extensively under controls of bidirectional provenance (LASP and SASP), mainly consisting of litharenite and sublitharenite, with low feldspar and high rock fragments volume, respectively. Strong compaction and carbonate cementation have largely destroyed primary porosity, while secondary pores constitute the main pore space in the Xu4 sandstone. The sediment provenance, sedimentary microfacies and A/S determine the clastic composition of sandstone, genetic type and thickness of sand bodies, furthermore affect the subsequent diagenetic variations. The larger the thickness of the sand body indicates a higher-energy depositional environment, coarser grain size, better sorting, and lower shaliness content of the sandstone during deposition period. This results in stronger resistance to compaction of the sandstone, which is conducive to the preservation of primary pores and dissolution of organic acids, resulting in better physical properties of sandstone. Among them, the physical properties of the sandstones of the Xu4s and Xu4x are better than those of the Xu4z; moreover, LASP's sandstone is superior to SASP. It is worth noting that subaqueous distributary channel sandstones with thickness exceeding 13 m in Xu4s and Xu4x from LASP represent favorable areas for high-quality reservoirs. The work can be used as a reference for the exploration and evaluation of similar tight sandstone gas in the world.

Linking diagenesis and reservoir quality to depositional facies in marginal to shallow marine sequence: An example from the Campano-Maastrichtian Gombe Sandstone, Northern Benue Trough, NE Nigeria

Fluvio-deltaic strata host significant volumes of hydrocarbons in different basins across the world, and are characterized by depositional heterogeneities and varying diagenetic overprints, owing to complex interactions of different depositional processes in diverse sub-environments. An understanding of the heterogeneities at different scales is critical to the prediction of reservoir quality distribution and architecture in these strata. The Campanian-Maastrichtian Gombe Sandstone is one of the targeted reservoirs in recent hydrocarbon exploration in the Northern Benue Trough, NE Nigeria. However, to date, the diagenetic and reservoir quality evolutions of the sandstone are not well understood. In this study, a multi-technique approach (consisting of thin-section petrography, scanning electron microscopy, and X-ray diffraction analyses) was employed to link depositional facies and diagenesis to reservoir quality. Results of the study indicate that the sandstones are principally arkosic and subarkosic in composition, and were deposited in lower delta front/prodelta, upper delta front/mouth bar, distributary channel, and interdistributary tidal bar/channel depositional environments. The sandstones experienced both shallow burial and some relatively of deep burial diagenesis. Kaolinite is the most abundant eogenetic cement in the sandstones, and was particularly found to be more abundant in the interdistributary tidal bar/channel and lower delta front/prodelta depositional environments due to their significant amounts of reactive detrital feldspars and muscovite precursors, respectively. The interdistributary tidal bar/channel and the distributary channel depositional facies were highly susceptible to siderite cementation, owing to their proximity to organic-rich mudstone lithofacies. Mechanical compaction was more intense in the lower delta front/prodelta than in the interdistributary tidal bar/channel sandstones, due to the presence of high clay matrix content in the former, which enhanced ductility and close packing. The interdistributary tidal bar/channel sandstones form the best reservoir quality because of the very low amounts of detrital pore-filling matrix (average 1%; n = 24) and less compaction compared to the lower delta front/prodelta, upper delta front/mouth bar and distributary channel sandstones. Grain-coating smectitic clays were best developed in the lower delta front/prodelta, upper delta front/mouth bar and distributary channel, whereas they are poorly developed in the interdistributary tidal bar/channel sandstones. The overall low volumes of deep burial, mesogenetic cements in the sandstones (e.g., illite, dickite, and quartz overgrowths) suggest that the Gombe Sandstone spent short residence time in mesogenetic realm before being uplifted. The results of this study would provide useful input data and understanding that can be utilized for forward diagenetic modelling and burial history construction for the subsurface Gombe Sandstone reservoir in the Northern Benue Trough and similar sedimentary formations elsewhere.

Implication Linkage among Microfacies, Diagenesis, and Reservoir Properties of Sandstones: A Case Study of Dongying Formation, Nanpu Sag, Bohai Bay Basin

The reservoir quality of sandstones is significantly impacted and transformed by sedimentation and diagenesis. It is necessary to clarify the internal relationship among them to precisely predict the sweet reservoir. In this study, five types of sedimentary microfacies are recognized through core observation and logging data: submerged distributary channel (fan delta), submerged interdistributary bay, submerged distributary channel (braided delta), distal bar, and turbidite fan. The major diagenetic processes, including compaction, cementation, and dissolution, have been analyzed based on petrography, scanning electron microscopy, and X-Ray diffraction. The dominant diagenetic cement includes calcite, smectite, kaolinite, illite, and I/S mixed-layer minerals, with small quantities of chlorite, pyrite, siderite, feldspar, and quartz cement. The reservoir quality is best in the submerged distributary channel (fan delta) sandstones, followed by submerged distributary channel (braided delta). Submerged interdistributary bay, distal bar, and turbidite fan are of poor reservoir quality. The grain size is the primary reservoir quality controlling factor, highly affected by sedimentary microfacies. Subsequent controls are diagenetic processes such as mechanical compaction, clay minerals formation, grain replacement, and dissolution that collectively influence the porosity and permeability.

Quantitative evaluation of tight oil reservoirs in the Chang 8 Member of the Yanchang Formation in southern Ordos Basin

The precise and quantitative characterization of reservoir properties is the key to efficient development of tight oil reservoirs. In this paper, taking the Chang 8 Member of the Yanchang Formation in the Shuimogou area in the southern Ordos Basin as an example, the sedimentary facies types, microscopic pore structures, diagenesis, influencing factors of physical properties, and hydrocarbon enrichment law of tight oil reservoirs are systematically studied. The research results show that the Chang 8 Member in the study area is a typical delta front subfacies deposit, including distributary channel and inter-distributary bay microfacies. The constructive diagenesis of the Chang 8 Member include dissolution, metasomatism and rupture; while the destructive diagenesis include mechanical compaction and cementation. The Chang 8 reservoir has entered the middle diagenetic stage A. The factors affecting the physical properties of tight oil reservoirs include deposition, compaction, cementation and dissolution. The secondary pores formed by dissolution account for 10–40% of the total surface porosity, with an average value of 24%. Local structures and sediments have significant control over hydrocarbon accumulation. The westward dipping tectonic setting of the northern Shaanxi Slope provides the basic conditions for the migration of oil and gas to the eastern updip areas. The changes of lithology and physical properties in the updip direction of the structure form the blocking conditions for the continued migration of oil and gas, which is conducive to the accumulation of oil and gas. In addition, the main oil reservoirs are mainly distributed along the distributary channel of the delta front, and most of the distributary channel sandstone is more than 20 m thick. The areas of thick sand body with multiple layers in the lateral direction is the main part of oil and gas accumulation. However, the reservoirs formed on the flanks of underwater distributary channels are generally thin and poorly sorted, which is not conducive to the accumulation of hydrocarbons.

Distribution of Upper Paleozoic Coal Seams in the Southeastern Ordos Basin

The Ordos Basin contains abundant coal resources in the Upper Paleozoic strata. They are shallowly buried or exposed in the periphery of the basin, forming recoverable coal mines and coalbed methane resources. For the deeply buried coal seams of the basin, however, due to complex changes and difficult mining, less attention was paid before. In recent years, with the exploitation of tight gas related to coal source rocks, a large amount of drilling and seismic data has been accumulated in the southeastern Ordos Basin, which makes it possible to study the distribution change and factors controlling Upper Paleozoic coal seams. Delineation of deep coal development and distribution is an important basis for coal, coalbed methane and related tight gas resources in the Upper Paleozoic. We use core, logging and seismic data to analyze the coal geophysics, thickness variation, paleotopography and sedimentary facies in the southeastern Ordos Basin. The lower part of the Shanxi Formation (Fm.) and the upper part of the Taiyuan Fm. in the southeastern part of the basin are characterized by low natural gamma ray, low density, high resistivity and high sonic logging values. As the coal velocity is very low and alternates with the high-velocity Taiyuan Fm. and Ordovician limestones, the two sets of coals below and above the Taiyuan limestones form an extremely strong-amplitude reflection in the seismic profile. The two sets of coals are cumulatively 1–10 m thick, averaging 2.4 m, and 4 m is the maximum induvial thickness. The individual coal seams thicker than 2 m cover an area of approximately 2500 km2, and the burial depths of the coal seam range from 2500 to 3000 m. The thick coal seams of the Taiyuan Fm. are mainly distributed proximally in the Ordovician paleokarst depression, while the Ordovician paleokarst depression without incised origin was deposited by distal coeval limestone rocks. There is a complementary relationship between the coal seams and the thickened zone of distributary channel sandstone in the Shanxi Fm.

Diagenetic evolution of the lower Yaojia Formation of Songliao Basin, China: Impact on reservoir quality

The oil-bearing layer in the first member of the Upper Cretaceous Yaojia Formation (K2y1) in the middle of the Changling Sag of the southern Songliao Basin, northeast China, has low-permeability and is deemed to be an unconventional reservoir. Therefore, better evaluation of reservoir quality and related controlling factors are crucial for the reduction in exploration risk and sustainable development. The current study integrates diagenesis, sequence stratigraphy, and sedimentary facies to decipher their influence on reservoir quality using core and thin section observations, X-ray diffraction, scanning electron microscopy, fluid inclusion, and carbon and oxygen isotope analyses. The K2y1 is composed of shallow delta sediments of transgressive system (TST) and lowstand system (LST) tracts. The depositional microfacies of the sandstones include TST deltaic front underwater distributary channels and sheet sands, and LST deltaic plain distributary channels, distributary channel intersections, and crevasse splays. The sandstones are dominantly fine-grained moderately sorted feldspathic litharenites and lithic arkoses with silt. Early diagenetic events included compaction, development of smectite, and alteration of feldspar and kaolinite whereas mesogenetic alteration included feldspar dissolution, precipitation of quartz and carbonate cements, and clay mineral alteration. The reservoir quality of the K2y1 sandstones is generally poor, exhibiting variations among different deltaic depositional facies within the LST and TST. The (underwater) distributary channel (intersection) sandstones, particularly those of the LST, have relatively higher reservoir quality owing to their larger grain size, better sorting, lower carbonate cement content, mixed-layer I/S and illite, and greater occurrence of chlorite.

Correlation of the lithostratigraphic facies relationships and depositional environments of the uppermost Silurian through Lower Devonian strata across the central Darling Basin, western New South Wales, SE Australia

This paper is focused on the detailed lithostratigraphic facies characterisation of the paleo-environments of the uppermost Silurian through Lower Devonian Winduck Interval strata from the Bancannia South 1, Pondie Range 1 and Booligal Creek 1 and 2 wells, across the central Darling Basin of western New South Wales. Based on data from both wireline logs and drill cores and cuttings, an interpretation of paleo-environments and evolution of the depositional facies is presented. Our approach involves two key interpretations. The first outcome was the delineation of the Winduck Interval section (maximum observed thickness of approximately 1475 m) and formal subdivision into lower and upper Winduck Intervals, based on well data from the central Bancannia Trough, southern Pondie Range Sub-basin and northwestern flank of the Blantyre Sub-basin. The second outcome is based on the internal lithostratigraphy of the Winduck Interval represented by 11 sedimentary lithofacies organised into three lithofacies associations (LFAs). LFA-X1 is interpreted as distributary-channel sandstone complex deposits, LFA-X2 is interpreted as distributary mouth bar with small-scale braided delta plain complex deposits and LFA-X3 is interpreted as tidal channel sands with proximal delta front complex deposits. Subsurface lithostratigraphic facies analysis relationships and depositional environments of the Winduck Interval section have implications for evaluating the hydrocarbon potential and may assist future exploration efforts in the central Darling Basin, especially the Bancannia Trough and Pondie Range Sub-basin. KEY POINTS A detailed lithostratigraphic facies characterisation of the paleo-environments of the uppermost Silurian–Lower Devonian Winduck Interval strata in the central Darling Basin is presented. Evaluation and interpretation of the Winduck Interval section in study area was subdivided into two informal stratigraphic units, a lower Winduck Interval and an upper Winduck Interval. Eleven distinct sedimentary lithofacies codes (LFs), which have been recognised in the internal lithostratigraphy of the Winduck Interval from the Bancannia South 1, Pondie Range 1 and Booligal Creek 1 and 2 wells, have been grouped into three lithofacies associations (LFAs). Lithostratigraphic facies analysis relationships and depositional environments of the Winduck Interval section has implications for evaluating the hydrocarbon potential and may assist future exploration in the Bancannia Trough and Pondie Range Sub-basin.

Seismic geomorphology of shallow-water lacustrine deltas in the Paleocene Huanghua Depression, Bohai Bay Basin, eastern China

The most important sedimentary facies comprising shallow-water lacustrine deltas are distributary channels, and these form abundant ribbon-shaped sand bodies that are excellent petroleum reservoirs. However, distributary channel sandstones are notoriously difficult to correlate and characterize in the subsurface based on well data alone. Here, we show how seismic geomorphology can be used to map lacustrine deltaic sedimentary successions in the First Member of the Kongdian Formation within the Zilaitun Oilfield, Huanghua Depression (China). Using core and wireline logs, this study identified distributary channels as the dominant sedimentary facies. Together with mottled gray-green or maroon mudstones and locally shingled progradational seismic reflection architecture, this indicates that deltas were built in a weakly-oxidized shallow-water lacustrine environment. Next, core observations were linked to seismic data to produce a successive series of stratal slices and root mean square amplitude attribute (RMSAA) maps. The RMSAA maps allowed distributary channel, mouth bar, and beach bar facies to be identified and mapped across the study area. Two patterns of distributary channels emerged from the data: (1) a distributive pattern with a lower concentration of distributary channels; and, (2) an anastomosing deltaic pattern with a high concentration of bifurcated distributary channels. Based on the seismic geomorphological analysis we show that the succession records three phases of delta evolution - retrogradation, aggradation, and progradation - which combine to form 3rd-order sequences. From a petroleum reservoir point of view, the distributive pattern yields narrower, thicker, and less connected sand bodies than the anastomosing pattern. This is consistent with observations from modern shallow-water lacustrine delta systems, providing independent support for the interpretation and highlighting the effectiveness of using seismic geomorphology.

Diagenesis and reservoir quality in high-resolution sandstone sequences: An example from the Middle Jurassic Ravenscar sandstones, Yorkshire CoastUK

A section of the Middle Jurassic Ravenscar sandstones exposed between Cloughton Wyke and Burniston Bay was studied to understand fine-scale variation in reservoir quality that can affect flow properties at the reservoir scale. The study has identified two 3rd-order depositional sequences consisting of twelve 4th-order high-resolution sandstone sequences deposited mostly in a lower delta plain. Although these sandstones exhibit a narrow range in porosity values, however, a sharp disparity exists in their permeability which can have implications at the scale of reservoir development. Diagenetic studies have revealed that mechanical compaction produced the same effect for most of these sandstones. However, dissolution and cementation vary, hence, are mostly responsible for the sharp contrast in their reservoir quality. The distributary channel sandstones developed during normal regression of the area in the FS9 sequence are considered the best reservoir sand bodies. However, these LST sandstones are compartmentalised by authigenic illite, quartz and kaolin into two reservoir segments. The upper portion lacks illite with resultant higher permeability values above 1100 mD (ave. 2217 mD) and classified as reservoir group 4. In contrast, the lower part contains illite with relatively lower permeability values in the range of 300–1100 mD (ave. 879 mD) and classified as reservoir group 3 together with the transgressive crevasse channel sandstones in the FS11 sequence. The presence of illite cement in these sandstones is linked to the occurrence of k-feldspar, hence, facilitated illitization of kaolin in contrast to group 4, which lacks this mineral. The other transgressive sandstones deposited in interdistributary bay, crevasse delta, back-barrier lagoon, lagoon, and estuarine environments which form reservoir groups 1 and 2 have permeability values in the range of 0–50 mD (ave. 6 mD) and 50–300 mD (ave. 184 mD) respectively. These sandstones have complex styles of diagenetic cement with variable higher volumes of total authigenic cement consisting of quartz, kaolin, illite, chlorite and calcite than reservoir groups 3 and 4, hence, have significantly reduced their reservoir potential. Therefore, for effective reservoir development of these sandstone reservoirs in the North Sea and elsewhere in the world with a similar geological setting, detailed reservoir characterisation involving linking lithofacies, sequence stratigraphy and diagenesis at high-resolution are essential for understanding the distribution of reservoir flow properties.

Facies analysis relationships depositional environments of the subsurface stratigraphy of the Snake Cave Interval in the Bancannia Trough, western Darling Basin, New South Wales, SE Australia

Abstract This work presents the first detailed research on the subsurface stratigraphy of the sedimentary Snake Cave Interval (late Early to late Middle Devonian) in the Bancannia Trough, the westernmost component of the Darling Basin, in western New South Wales, Australia. The study is based on data from wireline-log responses (especially gamma-ray and sonic logs), and core and cuttings descriptions in three exploration wells that are used to develop a model for depositional environments in the Bancannia Trough. This study provides re-evaluation and re-interpretation of the positions of the top and bottom boundaries of the Snake Cave Interval using distinct patterns changes in gamma-ray and sonic log signatures in relation to lithological characteristics in the Bancannia South-1, Bancannia North-1 and Jupiter-1 wells. The log facies interpretation of the Snake Cave Interval section enabled the detailed facies description of four lithostratigraphic units (A, B, C and D, in ascending order) in the study area. The Snake Cave Interval displays units with ten distinct sedimentary facies identified from core and then tied to the gamma ray log characteristics, forming facies associations that are visible in both vertical and lateral sections. Five facies associations have been identified: FA-I, braided fluvial multistory channel belt complex deposits; FA-II, meandering fluvial distributary channel sandstone complex deposits; FA-III, sandy braided fluvial distributary channels deposits; FA-IV, meandering fluvial deposits; and FA-V, fluvial channel-fill association with fluvio-tidal transition zone complex deposits. Detailed comparison of lithologic logs with wireline-log suite signatures (mainly gamma-ray logs) that are dominantly braided fluvial and meandering fluvial deposits, with minor fluvio-tidal transition zone complex deposits in this trough. This article presents a new depositional model that will promote exploration and discovery of potential petroleum resources and stratigraphic-trap configurations in this trough and adjacent regions of the Darling Basin.

Seismic geomorphology and sedimentology of a fluvial-dominated delta: Implications for the Neogene reservoirs, offshore Bohai Bay Basin, China

Fluvial-dominated deltas are common along modern and ancient coasts and act as important hydrocarbon reservoirs. In this paper, an integration of high-resolution three-dimensional seismic, well log, and core data are employed to investigate the seismic geomorphology, depositional facies, reservoir types, and controlling factors of fluvial-dominated delta deposition in the lower segment of the Minghuazheng Formation (N1mL), Bohai Bay Basin, China. Three typical seismic facies and seismic geomorphologic units are identified. Seismic facies 1 displays discrete high-amplitude reflection and a distinct U-shape incision. In plan form, this seismic facies shows a linear, dendritic, and sinuous morphology with high root-mean-square amplitudes. Seismic facies 2 occurs interspersed with seismic facies 1 and shows low-frequency and low-amplitude reflection. Seismic facies 3 shows a continuous high-amplitude reflection and uniform sheet-like morphology covering more than 10 km2 (>3.86 mi2). The N1mL was primarily deposited in the upper delta plain, lower delta plain, and delta front environments and is dominated by three major facies types: (1) distributary channel (seismic facies 1), (2) interdistributary bay (seismic facies 2), and (3) sheet sand (seismic facies 3). Among them, the distributary channel sandstones and sheet sandstones are the major reservoirs in the N1mL. Fluvial processes and lake level cycles were important factors in the development and distribution of reservoirs and traps in fluvial-dominated delta systems. Integration of the seismic geomorphology and a modern geomorphology investigation provide an effective way to predict the sandstone reservoirs and traps in fluvial-dominated delta systems.

Fluvial shoal water deltas: pre‐vegetation sedimentation through the fluvial–marine transition, Lower Cambrian, English Channel region

AbstractLower Palaeozoic fluvial systems tend to be more sand‐prone than those of later eras and the nature of coastal environments less certain. Field studies are presented that characterize the fluvial to marine transition over a distance of 80 km, in the Lower Cambrian of the Cotentin Peninsula, northern France. The sedimentary rocks are divided into six facies associations which represent deposition in proximal fluvial, distal fluvial, delta plain, delta front, pro‐delta and offshore carbonate bank environments. The basin fill is sandstone‐dominated and subdivided into three stratigraphic intervals. A 200 to 300 m thick basal interval contains very coarse‐grained fluvial sandstones deposited during a relative sea level lowstand. An overlying interval, 250 to 1500 m thick, is a facies mosaic. Fluvial strata in the north‐west pass laterally south‐east into deltaic and shallow marine pro‐delta sediments. The delta front deposits show repetitively stacked, upward‐coarsening parasequences, 8 to 10 m thick, which reflect the repeated progradation of lobate, fluvially‐dominated deltas onto a shallow marine shelf. The deltas formed following marine transgression and accumulated during a period of gradually rising relative sea level. An upper unit, 130 m thick, containing offshore stromatolitic and oolitic limestones, caps the study interval and represents deposition during a relative sea level highstand. The fluvial and delta distributary channel sandstones of the middle unit contain <1% mudstone. The cohesionless substrate determined that deltaic distributaries were predominantly braided in character and subject to common bifurcations which resulted in an ordered diminution of channel size and competence in a seaward direction. Terminal distributary channels show evidence of migratory levées and mouth‐bars and consistently delivered fine to medium‐grained sand to the delta front. The study highlights an example of pre‐vegetation deltaic sedimentation that was hydraulically organized and predictable, despite being fed by braided fluvial systems with high levels of peak discharge.

Sedimentary and diagenetic controls on reservoir quality of low-porosity and low-permeability sandstone reservoirs in Chang101, upper triassic Yanchang Formation in the Shanbei area, Ordos Basin, China

There are many reservoirs with low porosity and low permeability in the world, and many large oilfields have been found in these reservoirs. Unlike conventional reservoirs, the genesis of low-porosity and low-permeability reservoirs and the controls of relatively high-porosity and high-permeability zones are still unclear. This study takes the Chang101 reservoir sandstones of the Shanbei area in the Ordos Basin, China, a typical low-porosity and low-permeability, as an example. Chang101 sandstones are fine-grained to medium-grained, moderately to well sorted arkose and lithic arkose deposited in a delta plain environment. It is a typical low porosity and low permeability reservoir with an average porosity of 10.21% and an average permeability of 2.88 mD. Pore types mainly include primary intergranular pores and secondary dissolved pores. The main controls on the reservoirs are the sedimentary environment and diagenesis. The sedimentary environment determines the initial properties of the reservoir sandstones, including the material composition and texture that determine the primary porosity and permeability. Diagenesis further transforms the reservoir quality to develop heterogeneity in the porosity and permeability. The reservoir sandstones have undergone complex diagenetic processes and are currently in stage A of mesodiagenesis. Compaction accounts for the major reduction of the primary porosity of the sandstones. Different types of cementation have both positive and negative influences on reservoir quality. Carbonate cementation, quartz overgrowths and some authigenic clay cementation(i.e., I/S and illite) are major pore-filling components, that reduce the reservoir quality. An appropriate amount of chlorite resists compaction and prevents quartz overgrowth, but excessive chlorite coatings fill the pores and block the pore throats. The laumontite cement is not only resistant to compaction but also easily dissolved to form a large number of effective secondary pores. The dissolution of feldspar and rock fragments plays a role in contributing to connecting the isolated intergranular pores and increasing the secondary pores. Relatively high porosity and high permeability zones are deposited in high-energy environment, mainly delta plain distributary channel sandstones with more detrital quartz, fewer rock fragments and coarser grains than other sandstones, were deposited in high-energy environment. There is weaker compaction and less or no carbonate cement, with the preservation of the primary pores by chlorite coatings and an increase in the pores by the dissolution of laumontite cement.

Controlling factors of reservoir quality in low-permeability reservoir within a sequence stratigraphic framework: a case study of the lower Jurassic sandstones in the Northern margin of the Qaidam Basin, China

The Qaidam Basin, one of China’s most prolific petroliferous basins, is located in the northeastern corner of the Qinghai-Tibet Plateau. Using analytical data from cores, thin-sections and seismic data, and combined with the previous sequence stratigraphy research results, research on the reservoir characteristics was undertaken. The main controlling factors of reservoir quality in the sequence stratigraphy framework in the Lenghu area are discussed. The Lower Jurassic includes lowstand system tract (LST) and transitional systems tract (TST); the highstand systems tract are missing owing to tectonic uplift. The Lower Jurassic sandstones are composed of moderately to poorly sorted, subrounded sandstone representing several representative environments, including fan-delta and lacustrine facies. The main types of the Lower Jurassic sandstones are mostly feldspathic litharenite and lithic sandstone. The pore types of the Lower Jurassic sandstones are mainly secondary dissolution pores with few fracture types. The Lower Jurassic sandstones have experienced strong compaction that resulted in the loss of primary porosity. Carbonate cementation of the LST sandstones is greater than in the TST sandstones. Dissolution, which has an important role in reservoir development, is mainly caused by acidic fluids from the matured source rocks in the LST and meteoric water ingression from the top sequence boundary in the TST. Distributary channel sandstones adjacent to coal and mudstones are favourable reservoirs with turbidite sandstones adjacent to the sequence boundary less favourable reservoirs.

Sedimentology of a wave-dominated and tide-influenced deltaic system, upper Middle Miocene, southwestern Ulleung Basin, Korea

Abstract Integrated sedimentological and ichnological study of the gas-bearing, upper Middle Miocene strata in the southwestern Ulleung Basin has identified five main lithofacies, arranged in order from offshore and offshore transitional environments to coastal-plain channel settings. In each of repeated successions, the basal mudstone-dominated, offshore to offshore transitional facies are sharply overlain by upper shoreface sandstones that are, in turn, erosively overlain by distributary channel sandstones. In the open-marine environment, interbedded storm and interstorm deposits are predominant with sporadic flood-event beds containing a navichnia trace-fossil suite. Bioturbation style in this facies tends to reflect depositional energy variations, alternating between dwelling structures in the storm beds and feeding and grazing trace-fossil assemblages in the interstorm beds. In the channel deposits, flood-interflood couplets are commonly expressed as cross-bedded, almost non-bioturbated sandstones that alternate with bioturbated, tidally-formed, interbedded sandstone and mudstone. From this, the co-existence of storm-wave and tide-generated structures in the studied interval suggests that the depositional environment is a wave-dominated and tide-influenced deltaic setting, with the recorded tidal influence restricted to the channels, which have a higher level of heterogeneity because of tidal sedimentation during periods of low river discharge. Stratigraphically, the 400-m thick, upper Middle Miocene section shows an overall, coarsening-upward succession, representing 3rd-order falling-stage to lowstand deposits as a result of long-term basinal uplift. This succession can, in turn, be broken into four, 4th-order stratigraphic packages, each of which comprises a transgressive (T)-regressive (R) succession that is separated by a transgressive surface. These packages can be further subdivided into small-scale, upward-coarsening successions (5th-order stratigraphic packages of parasequence scale). Each small-scale succession is sharply overlain by offshore mudstone which reflects a rapid vertical facies transition, representing a marine flooding surface. Internally, each parasequence is characterized by a progradational facies succession wherein offshore mudstones are erosively overlain by upper-shoreface sandstones, forming sharp-based shoreface deposits that are likely to be produced during a forced regression, an interpretation supported by the presence of an omission trace-fossil suite on the erosion surface. In this context, the high-frequency parasequences represent short-term relative sea-level falls superimposed on the longer-term falling sea level due to a compressional tectonic regime.

Impact of sequence stratigraphy, depositional facies, diagenesis and CO2 charge on reservoir quality of the lower cretaceous Quantou Formation, Southern Songliao Basin, China

Abstract The fourth member of the Lower Cretaceous Quantou Formation in Southern Fuxin Uplift zone of the Southern Songliao Basin is typical tight oil reservoir which has recently been explored and produced. For effective exploration and development, the evaluation and control factors of the reservoir quality of the K1q4 sandstones should be thoroughly studied. The study focused on the control of the combination of sequence stratigraphy, depositional facies and diagenesis on the reservoir quality, and identified the impact of CO2 charging, based on the core and thin section observation, XRD, SEM, fluid inclusion and isotope testing and analysis. The fourth member of the Lower Cretaceous Quantou Formation consists of lowstand system tracts (LST), transgressive system tracts (TST) and highstand system tracts (HST). The K1q4 sandstones were mostly deposited in distributary channels and crevasse splays of a delta plain within the LST and TST, and underwater distributary channels and sheet sands of a delta front within the HST. The K1q4 sandstones, mostly silt to medium grained, medium to well sorted, and are mostly lithic arkoses and feldspathic litharenites, have undergone significantly diagenetic alterations, such as compaction, quartz cementation, unstable grains (feldspar and volcanic fragments) dissolution, carbonate cementation and clay minerals cementation. The reservoir quality of the K1q4 sandstones are poor with low porosity and permeability, showing variations among the different system tracts and depositional facies, and are influenced by the charging of CO2 and controlled by the combination of sequence stratigraphy, depositional facies and diagenesis. The sandstones with high reservoir quality are mostly deposited in high energy environments, including the (underwater) distributary channel sandstones, especially the LST distributary channel sandstones with the best reservoir quality among the LST, TST and HST sandstones, which can be ascribed to larger grain size and better sorting resulting in larger intergranular pore space after compaction, and less content of carbonate cements, illite and mixed-layer I/S, and more pore-lining chlorite and more effectively dissolution. The poor reservoir quality of sandstones from crevasse splay and sheet sand is due to the small grain size and bad sorting which resulted in extensive compaction and the resulting weak dissolution. The charging of CO2 has dual impact on the reservoir quality of the K1q4 sandstones. The CO2 charging can induce the dissolution of unstable detrital grains and the precipitation of dawsonite. The former create new secondary pore space, while the latter occupy the intergranular pore space. The CO2 charging is harmful to the reservoir quality when the dawsonite content exceeds a threshold of 5%.

Hierarchical Architecture of Sequences and Bounding Surfaces In A Depositional-Dip Transect of the Fluvio-Deltaic Ferron Sandstone (Turonian), Southeastern Utah, U.S.A.

Abstract: Depositional-dip-oriented transects through coastal-plain to nearshore marine domains are essential elements of generic sequence stratigraphic models for continental-margin successions. Many questions remain about the validity of these models, however, because well-constrained outcrop examples are sparse. Presented herein is the first detailed study of a 30-km-long, almost continuously exposed, depositional-dip transect through nonmarine and marine facies of the fluvio-deltaic Ferron Sandstone. This transect is connected to the southern end of a previously documented, 67-km-long, depositional-strike-oriented section. Excellent exposure reveals relationships among facies, stacking patterns, and bounding surfaces across three orders of sequences and various shoreline trajectories. Deposits of sequences are dominated by falling-stage systems tracts, suggesting a regime dominated by long, gradual relative falls in sea level punctuated by shorter relative rises. Widespread fluvial downcutting is generally absent. Distributary-channel sandstones pass down dip into broadly contemporaneous shoreface and delta-front sandstones. A composite sequence boundary divides the study interval into two composite sequence sets. Sequence sets I–III comprise eastward-dipping, offlapping clinoform sets defining a descending, regressive shoreline trajectory (falling-stage deltas). Sequence sets IV–VI overlie a thin transgressive deposit and consist of progradational to aggradational clinoform sets (highstand deltas). Key bounding surfaces are similar at all scales of the hierarchy: they are diachronous, cryptic, and pass down depositional dip into conformable surfaces where they are almost impossible to recognize as key surfaces. Differentiation between units and surfaces that develop at different hierarchical levels is possible only by integrating detailed outcrop observations with regional stratigraphic architecture.

Sedimentary characteristics and seismic geomorphologic responses of a shallow-water delta in the Qingshankou Formation from the Songliao Basin, China

With the aid of modern sedimentology, seismic geomorphology, and the comprehensive analysis of geophysical data and core data from more than 10 wells, the Qingshankou Formation has been divided into 3 long-term cycles and further into 6 medium-term cycles in the Qijia area, Songliao Basin. The 3 long-term cycles, SQ1, SQ2 and SQ3 correspond to the members Qing I, Qing II and Qing III of the Qingshankou Formation, spanning 11.5 Ma during the Late Cretaceous. The time scale of the medium-term cycles, which are subunits of the long-term cycles, is approximately 0.5–1.5 Ma with thicknesses ranging from 45 m to 116 m. A shallow-water delta that consists of high-maturity fine-grained sandstone developed in the Qingshankou Formation. The scour surfaces reflect strong flow action, and the parallel bedding and large-scale cross bedding are well developed. Distributary channel sandstones are well developed in the shallow-water delta depositional system, in which a single channel can be several meters thick. The cumulative thickness of the distributary channels can reach tens of meters across a large area, extending tens of kilometers to the center of the lacustrine basin. The development of the subaqueous distributary channel of the shallow-water delta is closely related to base level (lake level) changes. In the early stage of a rising half-cycle and the late stage of a falling half-cycle, the fluviation is significant and the well-developed channel sandstone can extend towards the center of the lacustrine basin more than 50 km, with large cumulative thicknesses of compound sandbodies (17–28 m) and high sandstone-to-strata ratios (78–88%). In the late stage of a rising half-cycle and the early stage of a falling half-cycle, the lake level is high and the fluviation is weak, resulting in a distributary channel extent of less than 30 km, small cumulative thicknesses of compound sandbodies (5–12 m) and low sandstone-to-strata ratios (22–45%). The delta front can be divided into the inner front and the outer front. The subaqueous distributary channels in the inner front have good continuity and extend further, whereas the subaqueous distributary channels in the outer front have poor continuity, and sheet sand is well developed. Therefore, a shallow-water delta generally shows features including (1) sedimentary structures formed by strong currents, (2) stacked fining upward rhythm, (3) subaqueous distributary channels well developed, (4) delta front divided further into an inner and an outer front. There was a relatively good linear relationship between the thickness of the sandbody based on the well data and the maximum amplitude of the seismic trace next to the well. The reddish amplitude of the seismic attribution suggests thicker channel sandbodies (4–9 m), whereas the light amplitude of the seismic attribution indicates thinner channel sandbody (1–4 m). Of the two typical sedimentary models for shallow-water deltas, delta plains develop well in humid climates and delta fronts develop well in arid climates, both of which are found in the Qijia area, Songliao Basin. Stratal slices from seismic geomorphology are applied to display the branchlike sandstone dispersal patterns of the shallow-water delta and the broad distribution characteristics. This study also notes that the delta front distributary channel sandstone, which is near an oil source and faults, would be a favorable exploration target.

Impact of sequence stratigraphy, depositional facies and diagenesis on reservoir quality: A case study on the Pennsylvanian Taiyuan sandstones, northeastern Ordos Basin, China

The Pennsylvanian Taiyuan Formation of the Shenmu Field in the northeastern Ordos Basin of China contains tight sandstone reservoirs that have recently been commercially produced. An evaluation of the reservoir quality of the Taiyuan sandstones will play a critical role in the development of the field. An investigation was conducted to explore possible effects of the sequence stratigraphy, depositional facies and diagenetic alterations on the reservoir quality of the Taiyuan sandstones based on core descriptions, the examination of thin-sections, and core plug and scanning electron microscope analyses. The Taiyuan Formation consists of lowstand system tracts (LST), transgressive system tracts (TST) and highstand system tracts (HST). The LST sandstones are principally medium-to coarse-grained and are interpreted to have been deposited in the dominant distributary channels of a delta plain. The early TST fine-to coarse-grained sandstones were largely deposited in the underwater distributary channel and mouth bar of a delta front, and the middle TST fine-to medium-grained sandstones were deposited in the shoreface to foreshore environment. The HST silt to medium-grained sandstones were mostly deposited in the mouth bar of a delta front and shoreface to foreshore. The Taiyuan sandstones, which are composed of sublitharenites, quartzarenites and litharenites, suffered from extensive compaction that resulted in a large loss of intergranular porosity before the sandstones were further cemented severely. The sandstone cements are mostly composed of illite, ankerite, calcite and quartz. The Taiyuan sandstones have a particularly low average core plug porosity of 7.41% and horizontal permeability of 0.916 mD, which results from that most of the pores of the sandstones are micropores and secondary pores with poor connectivity. The LST dominant distributary channel, early TST underwater distributary channel and proximal mouth bar, and HST shoreface to foreshore sandstones with relatively coarser grain sizes (mostly medium-to coarse-grained) have relatively higher porosities and permeabilities than the sandstones with relatively finer grain size, which is ascribed to a widespread early dissolution by meteor water leaching and lower content of cements within the coarser-grained sandstones. However, the LST dominant distributary channel sandstones had undergone a more effectively early dissolution and less carbonate cementation than the TST underwater distributary channel and proximal mouth bar and HST shoreface and foreshore sandstones had, which lead to the relatively best reservoir quality of the LST dominant distributary channel sandstones among the Taiyuan sandstones. The study revealed that the reservoir quality of the tight Taiyuan sandstones was controlled by sequence stratigraphy framework, depositional facies, grain size and diagenetic alterations together. The sequence stratigraphy framework played a key role in controlling the reservoir quality of the Taiyuan sandstones by governing the depositional facies and corresponding diagenetic products while the grain size affected the reservoir quality of the sandstones by controlling the content of cement within the sandstones.

Sedimentary facies analysis and paleogeographic significance of the latest Silurian to Early Devonian Winduck Interval in the Darling Basin, western New South Wales, Australia

This work presents the first detailed sedimentary facies analysis of the subsurface latest Silurian to Early Devonian Winduck Interval, based on four wells in the Blantyre and western Neckarboo sub-basins in the central part of the Darling Basin. The study integrates wireline logs, cores, cuttings and limited biostratigraphic data to determine the sedimentary sequence subdivision in the study area. Sedimentological analysis was applied, using characteristic wireline-log responses, and core and cuttings descriptions, to aid in the development of a depositional environment model for the Winduck Interval. Twenty-nine sedimentary facies are defined, forming five facies associations. These associations are characterised as distributary channel sandstone complex, distributary mouth bar, tidal channel sand, proximal delta front associated with mouth bars complex, and distal delta front to prodelta sediments. Tectonism influenced the paleogeography, although fault zones (a complex of normal and reverse faults) were important in determining sedimentary sequence thicknesses obtained from the wells. Tectonic subsidence was moderate, and accumulation rates were low. The results of sedimentary facies analysis of the Winduck Interval have implications for evaluating the hydrocarbon potential, and may assist extensive exploration efforts that are currently occurring in the Darling Basin within western New South Wales.