Chao Liang

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.

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.

Shale oil reservoir characteristics and enrichment in the Jiyang depression, Bohai Bay Basin, East China

Based on the observation of the well cores, thin section and FESEM, combined with X-ray diffraction, physical property testing and geochemical indicators, the reservoir characteristics and the controlling factors of the shale oil enrichment of the Ess 4–Esx 3 shale in the Jiyang depression were detailed analyzed. Studies show that carbonate and clay minerals are dominated in the shale. According to the triangle chart, the TOC content (2% and 4%), carbonate and clay minerals, nine lithofacies have been identified. The reservoir space types are rich in the shale, in which, the laminated fractures, recrystallization intracrystalline pores and organic pores are high-quality reservoir spaces. The shale oil enrichment is mainly determined by the hydrocarbon-producing potential and reservoir capacity. The hydrocarbon- producing capacity is controlled by the organic geochemistry indicators, especially the TOC content for the study area, and the thickness of the organic-rich shale. The reservoir capacity is mainly affected by the lithofacies, the TOC content and the structural activities. In addition, the shale oil production is influenced by the fracability of the shale, which is mainly controlled by the lithofacies, structural activities, formation pressure, etc. The shale oil reservoir evaluation should focus on the TOC content, the thickness of the organic-rich shale, lithofacies and structural factor.

Sedimentary characteristics and paleoenvironment of shale in the Wufeng-Longmaxi Formation, North Guizhou Province, and its shale gas potential

The Paleozoic Wufeng-Longmaxi shale is one of the main horizons for shale gas exploration in Sichuan Basin. Outcrop, core and thin section observations, X-ray diffraction analysis, trace element geochemistry and other methods have been used to understand the sedimentary characteristics and identify hydrocarbon source rocks in suitable sedimentary paleoenvironments in the Wufeng-Longmaxi shale in northern Guizhou Province. The thickness of the Wufeng-Longmaxi Formation ranges from 20 to 200 m and it was mainly deposited on a deep-water shelf. The TOC content is high, up to 5.75%. The main non-organic minerals are detrital quartz and clay minerals, with a little plagioclase feldspar, potassium feldspar, calcite, dolomite and pyrite. There is also biogenic microcrystalline quartz. Six lithofacies have been identified: siliceous shale, clay shale, calcareous shale, silty shale, carbonaceous shale, and muddy siltstone. Using biological Ba, V/(V+Ni), TOC, V/Cr, B, Sr/Ba and other indicators, we estimate primary productivity, redox conditions and paleosalinity and show that the early stage of Wufeng-Longmaxi deposition occurred under strong anoxic conditions, high paleosalinity and yielded a high TOC content and an excellent potential shale gas source. The anoxic environment was destroyed at the late stages of Wufeng-Longmaxi deposition, the TOC content decreased, so that it is likely to be a high quality source rock. Organic pores acted as the key reservoir space in the shales, and the pores are mainly mesopose, with most pore diameters less than 20 nm. The siliceous shale has high TOC content and brittle mineral (quartz) content making it an important exploration target for shale oil and gas exploration.

Classification of hydrocarbon-bearing fine-grained sedimentary rocks

Fine-grained sedimentary rocks are defined as rocks which mainly compose of fine grains (<62.5 μm). The detailed studies on these rocks have revealed the need of a more unified, comprehensive and inclusive classification. The study focuses on fine-grained rocks has turned from the differences of inorganic mineral components to the significance of organic matter and microorganisms. The proposed classification is based on mineral composition, and it is noted that organic matters have been taken as a very important parameter in this classification scheme. Thus, four parameters, the TOC content, silica (quartz plus feldspars), clay minerals and carbonate minerals, are considered to divide the fine-grained sedimentary rocks into eight categories, and the further classification within every category is refined depending on subordinate mineral composition. The nomenclature consists of a root name preceded by a primary adjective. The root names reflect mineral constituent of the rock, including low organic (TOC<2%), middle organic (2%4%) claystone, siliceous mudstone, limestone, and mixed mudstone. Primary adjectives convey structure and organic content information, including massive or limanited. The lithofacies are closely related to the reservoir storage space, porosity, permeability, hydrocarbon potential and shale oil/gas sweet spot, and are the key factor for the shale oil and gas exploration. The classification helps to systematically and practicably describe variability within fine-grained sedimentary rocks, what’s more, it helps to guide the hydrocarbon exploration.