Yingchang Cao

Characteristics and origin of abnormally high porosity zones in buried Paleogene clastic reservoirs in the Shengtuo area, Dongying Sag, East China

There are three abnormally high porosity zones developed in buried Paleogene nearshore subaqueous fan and sublacustrine fan clastic reservoirs at 2,800–3,200 m, 3,250–3,700 m and 3,900–4,400 m, respectively, within the Shengtuo area of the Dongying Sag. Here the porosity of reservoirs buried deeper than 4,000 m can still be greater than 20%. Investigation of these three abnormally high porosity (AHP) zones in the 3rd to 4th member of the Paleogene Shahejie Formation in the Shengtuo area was carried out with utilization of core observation, thin section identification, SEM observation, image analysis, core physical property testing and other technical methods. The results show that, the AHP zones in 2,800–3,200 m and 3,250–3,700 m are visible pores primary AHP zones dominated by significant primary intergranular pores (more than 50% of the total porosity), while secondary pores and micropores in authigenic clays may develop in some reservoirs. AHP reservoirs in the AHP zone of 3,900–4,400 m are dominated by micropores in matrix, visible pores are mainly grain dissolution pores but with low absolute content (< 1%), so this zone belongs to the micropores primary AHP zone. The genesis of the three AHP zones was studied to distinguish between porosity enhancement and porosity preservation. Our research shows that, in deeply buried clastic reservoirs in the Shengtuo area, mineral dissolution occurred in a relatively closed diagenetic system with high temperature and high salinity. Reservoir rocks underwent extensive feldspar dissolution, while detrital carbonate grains and carbonate cements show no evidence of extensive dissolution. Although significant feldspar dissolution pores developed, feldspar dissolution enhanced porosity only a little due to the precipitation of almost isovolumetric dissolution products in the nearby primary intergranular pores in forms of authigenic clays and quartz cements. Net enhanced porosity originating from feldspar dissolution is generally less than 0.25%. Thus, the subsurface dissolution has little impact on the mid-deep buried high porosity reservoirs. Reservoirs in braided channels of middle fans in sublacustrine fans and reservoirs in the middle-front of fan bodies of nearshore subaqueous fans provide the basis for the development of AHP zones. The shallow development of fluid overpressure and early hydrocarbon emplacement have effectively retarded compaction and carbonate cementation, so that the high porosity in the superficial layers is preserved in the mid-deep layers. These are the main controlling factors in the development of AHP zones.

Deep-water depositional mechanisms and significance for unconventional hydrocarbon exploration: A case study from the lower Silurian Longmaxi shale in the southeastern Sichuan Basin

The purpose of this work was to study the depositional mechanisms and significance of the Longmaxi shale in the Sichuan Basin in southern China. Seven lithofacies were identified based on the detailed observation of outcrops and cores using petrographic and scanning electron microscope examination of thin sections and other data analyses: (1) laminated calcareous mudstone, (2) laminated carbonaceous mudstone, (3) laminated silty mudstone, (4) laminated claystone, (5) laminated siliceous shale, (6) siltstone, and (7) massive mudstone. The laminated mudstone and laminated claystone originated from suspension deposition, and siliceous shale is associated with ocean upwelling, whereas massive mudstone and siltstone were primarily deposited by turbidity currents. The depositional mechanisms have a great effect on the source rock and reservoir properties. Suspension deposition near oceanic upwelling zones can provide favorable conditions for the production and preservation of organic matter and are thus conducive to the formation of high-quality source rocks (total organic carbon content up to 5.4%). The reservoir storage spaces are primarily interlaminated fractures and organic pores with good physical reservoir properties (high porosity, permeability, and brittle mineral content). Turbidity currents may carry a large quantity of oxygen to the seafloor, resulting in the oxidation of organic matter, which is unfavorable for its preservation. The lithofacies formed by turbidity currents have relatively low total organic carbon contents (average: <1%). Structural fractures and intergranular pores are the primary storage spaces that are present in the reservoir. In summary, organic-rich shale and siliceous shale that was deposited from suspension near upwelling zones are key exploration targets for shale oil and gas. The widely distributed, multilayer, tight sandstone is important in the exploration for tight oil. A better understanding of the deposition mechanism and its effect on oil reservoirs may assist in identification of favorable areas for exploration.

Feldspar dissolution, authigenic clays, and quartz cements in open and closed sandstone geochemical systems during diagenesis: Typical examples from two sags in Bohai Bay Basin, East China

Feldspar dissolution and precipitation of clays and quartz cements are important diagenetic reactions affecting reservoir quality evolution in sandstones with detrital feldspars. We examined two sets of sandstone reservoirs to determine whether the sandstone diagenetic systems were open or closed to the mass transfer of products from feldspar dissolution and its impact on reservoir quality. One of the reservoirs is the Eocene fan delta sandstone buried 2.5–4.0 km (1.5–2.5 mi) below sea level (BSL) in the Gaoliu (GL) area of the Nanpu sag, and the other is the Eocene subaqueous fan sandstone buried 1.5–4.5 km (1–2.8 mi) BSL in the Shengtuo (ST) area of the Dongying sag. Both sandstones consist mainly of lithic arkoses and feldspathic litharenites, and have secondary porosity formed by dissolution of feldspars. In the GL sandstones, the absolute amounts of authigenic clays and quartz cements (generally 125°C [257°F]). The low abundance of authigenic clays and quartz cements, and low pore-water salinity indicate that much of the , , and released from leached K-feldspars were exported from the GL sandstone system. And the extensive feldspar dissolution enhanced much porosity and permeability. In contrast, the ST sandstones with secondary pores formed by feldspar dissolution generally contain authigenic clays (kaolinite and illite) and quartz cements with almost identical volume of secondary pores. Kaolinite dominates in the ST sandstones at shallower depth (3.1 km [2 mi] BSL) where temperature exceeds 125°C (257°F). The presence of abundant clays and quartz cements indicates that and released from leached feldspars were retained in the ST sandstone system. The dominance of authigenic illite at greater depth indicates that sufficient should have been retained within the sandstones for occurrence of illitization of kaolinite and feldspars. Secondary porosity in thin sections can be up to 3%, but little porosity ( The diagenetic difference between the GL and the ST sandstones can be interpreted by assessing pore-water evolution in these two areas. The current pore waters with low salinity and negative hydrogen isotopic compositions in the GL sandstone system indicate the significant impact of meteoric water, whereas the current pore waters with high salinity and the paleofluids with positive oxygen isotopic compositions in the ST sandstone system indicate little trace of meteoric water. Access of meteoric freshwater to the GL area probably occurred during the late Oligocene to Neogene through widely developed faults in the Paleogene and Neogene strata. The low-salinity water could have been responsible for flushing of solutes derived from feldspar dissolution. As such, diagenesis in the GL sandstones is considered to have occurred in an open geochemical system, whereas with limited faults and high water salinity, the ST sandstones acted as a closed geochemical system where precipitation of kaolinite, illite, and quartz cements occurred following dissolution of feldspars.

Mechanism of diagenetic trap formation in nearshore subaqueous fans on steep rift lacustrine basin slopes—A case study from the Shahejie Formation on the north slope of the Minfeng Subsag, Bohai Basin, China

Diagenetic traps in conglomerate in nearshore subaqueous fans in the steep slope zones of rift basins have been important exploration targets for subtle reservoirs in eastern China. However, the mechanism of how those traps were formed is not clear, which inhibits further exploration for this type of reservoir. In order to solve the problem, we take as an example nearshore subaqueous fans in the upper part of the fourth member of the Shahejie Formation (Es4s) on the north slope of the Minfeng Subsag in the Dongying Sag. Combining different research methods, such as core observation, thin section examination, scanning electron microscope (SEM) observation, fluid-inclusion analysis, carbon and oxygen isotope analysis of carbonate cements, and analysis of core properties, we studied the genetic mechanisms of diagenetic traps on the basis of diagenetic environment evolution and diagenetic evolution sequence in different sub/micro-facies. Conglomerate in Es4s in the north Minfeng Subsag experienced several periods of transition between alkaline and acidic environments as “alkaline-acidic-alkaline-acidic-weak alkaline”. As a result, dissolution and cementation are also very complex, and the sequence is “early pyrite cementation / siderite cementation / gypsum cementation / calcite and dolomite cementation—feldspar dissolution / quartz overgrowth—quartz dissolution / ferroan calcite cementation / ankerite cementation / lime-mud matrix recrystallization / feldspar overgrowth—carbonate dissolution / feldspar dissolution / quartz overgrowth / pyrite cementation”. The difference in sedimentary characteristics between different sub/micro-facies of nearshore subaqueous fans controls diagenetic characteristics. Inner fan conglomerates mainly experienced compaction and lime-mud matrix recrystallization, with weak dissolution, which led to a reduction in the porosity and permeability crucial to reservoir formation. Lime-mud matrix recrystallization results in a rapid decrease in porosity and permeability in inner fan conglomerates in middle-to-deep layers. Because acid dissolution reworks reservoirs and hydrocarbon filling inhibits cementation, reservoirs far from mudstone layers in middle fan braided channels develop a great number of primary pores and secondary pores, and are good enough to be effective reservoirs of hydrocarbon. With the increase of burial depth, both the decrease of porosity and permeability of inner fan conglomerates and the increase of the physical property difference between inner fans and middle fans enhance the quality of seals in middle-to-deep layers. As a result, inner fan conglomerates can be sealing layers in middle-to-deep buried layers. Reservoirs adjacent to mudstones in middle fan braided channels and reservoirs in middle fan interdistributaries experienced extensive cementation, and tight cemented crusts formed at both the top and bottom of conglomerates, which can then act as cap rocks. In conclusion, diagenetic traps in conglomerates of nearshore subaqueous fans could be developed with inner fan conglomerates as lateral or vertical sealing layers, tight carbonate crusts near mudstone layers in middle fan braided channels as well as lacustrine mudstones as cap rocks, and conglomerates far from mudstone layers in middle fan braided channels as reservoirs. Lime-mud matrix recrystallization of inner fan conglomerates and carbonate cementation of conglomerates adjacent to mudstone layers in middle fan braided channels took place from 32 Ma B.P. to 24.6 Ma B.P., thus the formation of diagenetic traps was from 32 Ma B.P. to 24.6 Ma B.P. and diagenetic traps have a better hydrocarbon sealing ability from 24 Ma B.P.. The sealing ability of inner fans gradually increases with the increase of burial depth and diagenetic traps buried more than 3,200 m have better seals.

Sedimentary characteristics and origin of lacustrine organic-rich shales in the salinized Eocene Dongying Depression

Lacustrine organic-rich shales are well developed within the Eocene Dongying Depression in the Bohai Bay Basin in eastern China and across Southeast Asia. Understanding the sedimentation of these shales is essential to the study of depositional processes, paleoenviron ment, and paleoclimate reconstruction. This study investigates the sedimentary characteristics and formation mechanisms of lacustrine shales in the upper fourth member of the Eocene Shahejie Formation (Es4s) within the Dongying Depression based on thin sections and field emission scanning electron microscope (FESEM) observations of well cores combined with X-ray diffraction and geochemical indicators. Six litho facies were identified: (1) laminated calcareous mudstone, (2) laminated dolomitic mudstone, (3) laminated clay mudstone, (4) laminated gypsum mudstone, (5) massive mudstone, and (6) siltstone. The organic matter in the Es4s shale is mainly type I and type II kerogens, as well as a small proportion of type III kerogen. On the basis of lithofacies associations, paleosalinity values, redox properties, and terrigenous inputs, the lower Es4s shale can be divided into six intervals from bottom to top, numbered I, II, III, IV, V, and VI. The thickness of each interval ranges from several meters to more than 10 m, reflecting highfrequency oscillations in the environment of the lake basin, markedly different from a relatively stable marine environment. The laminated mudstones are characterized by fine grain sizes, scarce large terrigenous debris (quartz and feldspar), and compositions that are rich in pyrite and sapropelic organic matter. These features indicate that these lithofacies were deposited out of suspension in a quiet water body characterized by a relatively low rate of deposition. The characteristic laminae of these litho facies indicate subtle differences in depositional processes. The laminated gypsum mudstone was likely deposited in an evaporative environment, because its formation would have consumed Ca2+ and SO4, promoting the depo si tion of a laminated dolomitic mudstone. In contrast, laminated clay mudstone was deposited in a manner that increased the volume of small terrigenous materials. Deposi tion of this lithofacies was controlled by the nature of the water body, paleoclimate, and terrigenous inputs. Laminated mudstones are dominant in the lower Es4s shale, suggesting that suspension was the main depositional process leading to formation of the lower Es4s shale. In contrast, the massive mudstones were likely rapidly deposited asso ciated with siltstone as the result of finegrained turbidites. The lower Es4s shale was formed in a depositional environment composed of a saline, medium-depth lake under anoxic conditions, with limited terrigenous inputs. The depositional process included suspension and turbidity currents. The high salinity is suggested to be related to a marine transgression, which may have been facilitated by a rise in sea level caused by global warming in the early Eocene, together with the large-scale tectonic activity of East Asia. Seawater input affected the lithofacies, influenced lake water body conditions, triggered turbidity currents, and prompted the accumulation of organic matter. The deposition of the Es4s shale in the Dongying Depression may help us to understand the deposition of lacustrine shale, paleoclimate reconstructions for the Eocene, and the tectonic activity of East Asia. INTRODUCTION Fine‐grained sedimentary rocks mostly contain grains that are smaller than 62.5 mm and comprise approximately two thirds of the stratigraphic record (Aplin et al., 1999; Stow and Mayall, 2000; Tucker, 2001; Aplin and Macquaker, 2011). Of these rocks, mudstones consist of a variable mixture of clay minerals, quartz, feldspars, carbonates, sulfides, amor‐ phous material, and organic matter (Macquaker and Adams, 2003; Potter et al., 2005; Milliken, 2014). Organic‐rich mudstones, in particular, act as important petroleum sources, reservoirs for shale oil and gas, and seals in conventional reservoirs (Schieber, 1999; Andersson and Wor‐ den, 2004; Bowker, 2007; Jarvie et al., 2007; Abouelresh and Slatt, 2012). Because of its ap‐ parent homogeneity and the limitations of ultra‐ microscopic experimental equipment, shale has been often overlooked in sedimentological stud‐ ies over recent decades (Arthur and Sageman, 1994; Schieber, 1999; Potter et al., 2005; Jiang et al., 2013). However, because of extensive and successful deep‐water hydrocarbon explora‐ tion, especially for shale oil and gas (Hill et al., 2007; Loucks and Ruppel, 2007; Slatt, 2007; Kuang et al., 2012), deep‐water sedimentation has become the focus of considerable atten‐ tion, including the depositional environments, transport, and depositional processes associ‐ ated with organic matter preservation in black shales ( Macaquaker et al., 2007, 2010b; Piper and Calvert, 2009; Aplin and Macquaker, 2011; Konitzer et al., 2014; Liang et al., 2016). The depositional processes leading to black shales mostly involve pelagic and hemi pelagic settings, turbidity currents, debris flows, slides, and wave‐enhanced sediment gravity flows (Stow and Bowen, 1980; Stow and Mayall, 2000; Soyinka and Slatt, 2008; Bouma and Stone, 2000; Macquaker et al., 2010a; Ghadeer GSA Bulletin; January/February 2018; v. 130; no. 1/2; p. 154–174; https://doi.org/10.1130/B31584.1; 16 figures; 3 tables; published online 29 August 2017. liangchao0318@163 .com For permission to copy, contact editing@geosociety.org

Shale oil potential of lacustrine black shale in the Eocene Dongying depression: Implications for geochemistry and reservoir characteristics

ABSTRACT The geochemistry and reservoir characteristics of the lacustrine shale in the Eocene Dongying depression are described in detail based on thin-section and field-emission–scanning electron microscope observations of well cores combined with x-ray diffraction, physical property testing, and geochemical indicators. The Eocene Shahejie (Es) Formation Es4s–Es3x shale member is predominantly carbonate, clay minerals, and quartz. Six lithofacies were identified: (1) laminated limestone (organic-rich laminated limestone and organic-poor laminated limestone), (2) laminated marl, (3) laminated calcareous mudstone, (4) laminated dolomite mudstone, (5) laminated gypsum mudstone, and (6) massive mudstone. The Es4s–Es3x shale samples from three cored wells had total organic carbon (TOC) contents in the range of 0.58 to 11.4 wt. %, with an average of 3.17 wt. %. The hydrocarbon generation potential (free hydrocarbons [S1] + the hydrocarbons cracked from kerogen [S2]) values range from 2.53 to 87.68 mg/g, with an average of 24.19 mg/g. The Es4s–Es3x shale of the Dongying depression has a high organic-matter content with very good or excellent hydrocarbon generation potential. The organic maceral composition is predominantly sapropelinite (up to 95%). The hydrogen index (being S2/TOC) versus the maximum yield temperature of pyrolysate ( T max ) indicates that the organic matter is predominantly type I kerogen, which contains a high proportion of convertible organic carbon. The Es4s–Es3x shale is thermally mature and within the oil window, with the vitrinite reflectance values ranging from 0.46% to 0.74% and the T max value ranging from 413°C to 450°C, with the average being 442°C. The shale contains interparticle pores, organic-matter pores, dissolution pores, intracrystalline pores, interlaminar fractures, tectonic fractures, and abnormal-pressure fractures. The primary matrix pore storage is secondary recrystallized intercrystal pores and dissolution pores that formed during thermal maturation of organic matter. The TOC content and effective thickness of the organic-rich shales are the primary factors for hydrocarbon generation. The reservoir capacity is related to the scale, abundance, and connectivity of pore spaces, which are controlled by the characteristics of the lithofacies, mineral composition, TOC content, and microfractures.

Diagenesis and porosity-permeability evolution of low permeability reservoirs: A case study of Jurassic Sangonghe Formation in Block 1, central Junggar Basin, NW China

Abstract Based on core observation, thin section examination, cathode luminescence analysis, scanning electron microscopy, fluid inclusions, carbon and oxygen isotope, mercury penetration, porosity-permeability test and other analytical methods, combined with the histories of burial evolution, organic matter thermal evolution and hydrocarbon charge, the diagenesis and porosity-permeability evolution are studied of low-permeability reservoirs of Jurassic Sangonghe Formation in Block 1 of central Junggar Basin. The matching relation between reservoir porosity-permeability evolution and hydrocarbon accumulation history is analyzed. The diagenetic environment evolution of the reservoir in the study area is early alkaline, interim acid and late alkaline, forming the diagenetic sequence of chlorite membrane precipitation, early calcite cementation, feldspar dissolution accompanied by quartz overgrowth and authigenic kaolinite precipitation, anhydrite cementation, late period ferrocalcite and ankerite cementation, a small amount of pyrite cementation. Generally, compaction occurs throughout the whole burial process. According to the matching relation between reservoir porosity-permeability evolution and hydrocarbon accumulation history, the Jurassic Sangonghe Formation has three genetic types of low permeability reservoirs: densification after hydrocarbon accumulation, with the best exploration potential; densification during the hydrocarbon accumulation, with medium exploration potential; densification before the hydrocarbon accumulation, with the poorest exploration potential.

Origin of carbonate cements with implications for petroleum reservoir in Eocene sandstones, northern Dongying depression, Bohai Bay basin, China

The Eocene Es4s interval is an important petroleum reservoir of sublacustrine fan dominated at the depths of 2500–4000 m in Dongying depression, Bohai Bay basin. Based on core observation, three types of carbonate-cemented beds have been identified and commonly contain ferroan calcite and ankerite predominantly and less calcite and dolomite. Precipitation temperatures range from 34.6 to 72.8℃ for calcite and dolomite cements, and from 110 to 153℃ for ferroan calcite and ankerite cements. The high δ13CPDB values (−0.65 to +5.59‰) for calcite and dolomite suggest that dissolved inorganic carbon, derived from methanogenic fermentation of organic matter in adjacent mudstones. The low δ13CPDB values (+1.04 to +3.29‰) for ferroan calcite and ankerite probably indicate a mixture of carbon derived from decarboxylation of organic acid as well as from the dissolution of early formed carbonate cements. High plug porosity is mainly developed at the central section of sandstones vertically and the porosity decreases sharply toward the top and base of the sandstones due to extensively carbonate-cemented beds. The carbonate-cemented beds varies from 0.02 to 0.5 m in thickness and might extend from tens to hundreds meters laterally. It could be served as fluid-flow barriers and seals for petroleum, and result in reservoir deterioration and significant heterogeneity.

A new discovery of The Early Cretaceous Supercritical Hyperpycnal Flow Deposits on Lingshan Island, East China

Understanding the dynamics of sediment gravity flows is of great importance to correctly interpret their related deposits. The discovery of supercritical sediment gravity flows provides some new viewpoints for the explanation of controversial sediment gravity flow deposits. However, the dynamics, formation, evolution processes of supercritical sediment gravity flows and their recognition criteria from their associated deposits are still worldwide controversial. The supercritical hyperpycnal flow deposits recognized in the upper part of Early Cretaceous Lingshandao Formation provide a rare opportunity to understand their sedimentary characteristics. This work is aimed at documenting the typical sedimentary structures associated with the supercritical hyperpycnal flow, and discussing the vertical stacking and its relationship with flow evolution.

Characteristics and formation mechanisms of large volcanic rock oil reservoirs: A case study of the Carboniferous rocks in the Kebai fault zone of Junggar Basin, China

Volcanic hydrocarbon reservoirs are rare and may be overlooked. The Carboniferous volcanic rocks of the Kebai fault zone in the western Junggar Basin contain hydrocarbon (HC) reservoirs in volcanic rock with proven oil reserves of 9.76 × 108 bbl that have a complex filling history. We have investigated the lithology and properties of these volcanic rock HC reservoirs as well as diagenesis and control of faults and fractures in oil reservoirs. The lithology of these Carboniferous volcanic rocks is primarily andesite and tuff. Also present were volcanic breccia and metamorphic rock in addition to rhyolite, felsite, diabase, and granite in the volcanic lava. On the basis of microscopic examination, five types of pores and fractures were observed: (1) fracture–dissolved phenocrystal pore, (2) fracture–intergranular pore, (3) fracture–gas pore, (4) fracture–dissolved intragranular pore, and (5) fracture–dissolved matrix pore. The fractures in these rocks are a significant factor in connecting the pores. Diagenetic processes that control reservoir quality include compaction, filling of pores and fractures, cementation, metasomatism, and grain dissolution. The volcanic reservoirs show a variety of lithologies, and oil has been discovered in all types of Carboniferous rocks. The controlling factors for oil distribution in these Carboniferous volcanic rocks are faulting, fracture development, and degree of weathering when they were subaerially exposed in the Permian. The area in which these faults and fractures developed is the primary area of oil enrichment with high yields. The objectives of this study were to (1) describe the characteristics of different types of volcanic rocks and reservoirs found in this basin and (2) characterize the diagenetic history of these rocks and document how diagenesis controls porosity and permeability.