Petrologic Characteristics and Genetic Model of Lacustrine Lamellar Fine

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PETROLEUM EXPLORATION AND DEVELOPMENT Volume 42, Issue 5, October 2015 Online English edition of the Chinese language journal

Cite this article as: PETROL. EXPLOR. DEVELOP., 2015, 42(5): 656–666. RESEARCH PAPER

Petrologic characteristics and genetic model of lacustrine lamellar fine-grained rock and its significance for shale oil exploration: A case study of Permian Lucaogou Formation in Malang sag, Santanghu Basin, NW China

LIU Bo1, 2,*, LÜ Yanfang1, MENG Yuanlin1, LI Xinning3, GUO Xiaobo4, MA Qiang3, ZHAO Wanchun1 1. Accumulation and Development of Unconventional Oil and Gas, State Key Laboratory Cultivation Base Jointly-constructed by Heilongjiang Province and the Ministry of Science and Technology, Northeast Petroleum University, Daqing 163318, China; 2. “Fault Deformation, Sealing and Fluid Migration” Science and Technology Innovation Team in Colleges and Universities of Heilongjiang, Daqing 163318, China; 3. PetroChina Turpan-Hami Oilfield Company, Hami 839009, China; 4. School of Earth Sciences and Engineering, Xi’an Shiyou University, Xi’an 710065, China

Abstract: Taking the Permian Lucaogou Formation in the Malang sag, Santanghu Basin as an example, by using petrological methods such as high resolution core image scanning, conventional thin section, scanning electron microscope and energy spectrum analysis and geochemical tests such as trace elements and biomarker compounds, the petrologic features and sedimentary origin of the lamellar organic rich fine-grained rocks of lake facies were investigated, and its significance for shale oil and tight oil exploration was analyzed. The results of the study show that there are four types of laminae, siliciclastic enrichment laminae, carbonate enrichment laminae, tuffaceous enrichment laminae and organic matter enrichment laminae, which can form three kinds of layer combinations. Organic matter is laminar enrichment or dispersed in the carbonate laminae and tuff laminae. Stratification of ancient lake water was formed in the closed saline lake sedimentary systems with insufficient continental clast supply, and the activity of warm water at the lake bottom and the monsoon climate worked jointly to control enrichment of organic matter and formation of lacustrine lamina fine-grained rocks. This kind of fine-grained rocks have higher potential of hydrocarbon generation due to high abundance of organic matter, are rich in reservoir space due to the existence of micro-pores in carbonate laminae and micro-cracks between the laminae, and suitable for fracturing because of high brittle mineral content. They have favorable conditions for shale oil and tight oil accumulation, and are significant for exploration.

Key words: lamellar fine-grained rock; laminae combination; organic matter enrichment mode; petrologic characteristics; genetic model; shale oil and gas; tight oil and gas

1. Concept and characteristics with dispersed organic matter[5], for example, the third member of the Palaeogene Shahejie Formation in Dongying sag in Fine-grained rock refers to sedimentary rocks composed mainly of clay and silt less than 62 μm in grain size [1]. They Bohai Bay Basin, northern China, the laminated and organic- contain not only clay minerals, but also silt, carbonates, rich shale at the lower part of the member has better hydro- organic matter etc[2]. Laminae are the thinnest and the smallest carbon-generation capacity than the central part of the mem- [6] unit of original sedimentary layers recognizable from sedi- ber with scattered organic matter . In addition, when meeting ments or sedimentary rocks. There are many types of laminae, certain geological conditions, laminated mud shale can form such as lacustrine varves[3], marine varves[4] etc. In recent oil shales with industrial value, for example, the Jurassic [7] years, more and more researchers found out that shale with tasmanite comprising of marine algae in Alaska and the alternating dark and light laminae had richer organic matter Ordovician kukersite consisting almost entirely of telalginite and thus higher hydrocarbon generation potential than shale derived from Gloeocapsomorpha in Estonia[7]. Other examples

Received date: 26 Mar. 2015; Revised date: 25 Jul. 2015. * Corresponding author. E-mail: [email protected] Foundation item: Supported by the National Natural Science Foundation (41472125); National Natural Science Foundation for the Youth (41202101); Heilongjiang Natural Science Foundation for the Youth (QC 2015043); and Training Plan for Young and Innovative Talents from Undergraduate Schools in Heilongjiang Province (UNPYSCT-2015077). Copyright © 2015, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved. LIU Bo et al. / Petroleum Exploration and Development, 2015, 42(5): 656–666 include the dark lamellar oil shales made up of dead red algea and gas exploration in shale or other tight rocks (Fig. 1). in Qingbaikou system of the Neoproterozoic in Hebei 2. Petrological characteristics of lacustrine Province (TOC: 21.41%−22.91%, oil content: 5.29%−10.57%)[8], lamellar fine-grained rock and Shehejie Formation oil shales with organic laminae of algae, clay, and carbonates in Dongying sag of Bohai Bay Variable in lithofacies, complicated in mineral composition, Basin (TOC: 2%−8%)[9]. The two oil shales in China share the and strong in heterogeity, lacustrine fine-grained rocks are following features: (1) They contain laminae of less than generally formed in shallow lake or semi-deep lake – deep millimeter in thickness and of various types, including terri- lake where water power is weak. Laminae are the basic units genous clastics, crystal fragment, tuffaceous, and organic of fine-grained rocks and appear as silt, carbonate, organic laminae; (2) Organic-rich laminae alternate with organic–lean ones depending on specific sedimentary environment and laminae, and the two kinds of laminae differ greatly in TOC climate. Lacustrine fine-grained rocks usually consist of value, sometimes, as high as 10 to 30 times[10]; and (3) Dark interbedded and cycled laminae of different minerals. organic laminae give off strong yellow fluorescence under Generally 0.01 mm and 0.50 mm thick, laminae are different microscope, and are held in between mud, silt and carbonate in color, mineral composition, grain size, structure and genesis laminae with weak fluorescence. In this study, we performed (Fig. 2). systematic petrology study on fine-grained rock samples from 2.1. Basic laminae types the Permian Luocaogou Formation in Malang sag of Santang- hu Basin through high-resolution imaging scanning, thin Analysis shows that there are four major types of laminae section observation, cathode luminescence, SEM and energy in the study area: siliciclastic, carbonate, tuffaceous and orga- spectrum analysis to reveal its genesis and significance to oil nic-rich ones.

Fig. 1. Schematics of structures in Santanghu Basin and the location of the study area.

Fig. 2. High-resolution scanning and microscopic images of lacustrine lamellar fine-grained rocks in Lucaogou Formation of Malang sag. (a) Core scanning image showing alternative dark and bright laminae, Well N-122, 2 589.58−2 589.63 m; (b) SEI image showing minerals and organic matter of different colors in directional arrangement in laminae, Well N122, 2 589.60 m; (c) Photo taken with plane-polarizers showing vitrinite and inertinite strips lined up along bedding, Well N122, 2 589.00 m; (d) Photo taken with cross-polarizers showing micrite calcite/dolomite with white interference color, some recrystallized into fine grains and formed couplets with siliciclastics, Well N122, 2 589.60 m.

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Siliciclastic laminaes are usually organic-rich and dark but continuous and directional in distribution; pyrite appearing in would light up with a gympsum plate inserted. Mostly 15 to strawberry-shape in layered clay, is complete in crystal form, 50 μm thick, they contain mainly silty feldspar, quartz and some indicating strong reducing environment during its deposition. clay minerals, and small amount of ferrodolomite and pyrite Carbonate laminae are bright and stable even with gym- (Fig. 3a), in which clay minerals, mostly illite (80%), are psum plate inserted. Usually between 80 and 300 μm thick,

Fig. 3. Photos of typical thin sections of various laminae in Luocaogou Formation of Malang sag. (a) Image taken with cross-polarizers, showing dark and organic-rich laminae consisting of feldspar, quartz and small amount of clay, and bright laminae of micrite and fine-grained dolomite, Well N122, 2 590.20 m; (b) Image taken with cross-polarizers, dark laminae consisting of feldspar, quartz and small amount of clay, and bright laminae of micrite and fine-grained dolomite with small amount of intergranular clay and organic matter, Well M7, 2 064.80 m; (c) Image taken with cross-polarizers showing crystal feldspar pyroclast, Well M6, at depth of 3 137.80−3 137.92 m, (d) Image taken with cross-polarizers showing crystal feldspar pyroclast, feldspar number is determined at 36, Well M9, 3 607.00− 3 607.10 m; (e) Image taken with plane-polarizers, showing vitrinite lumens filled up with resinite presenting as laminae or single layer under lens, Well N122 2, 590.30 m; (f) Sharing the same field of vision with (e), image showing yellow fluorescence, indicating abundant mineral bitumen matrix. − 658 − LIU Bo et al. / Petroleum Exploration and Development, 2015, 42(5): 656–666 they are composed of fine-grained microcrystalline calcite and thick, contain mainly amorphous sapropelinites mostly with dolomite (less than 30 μm), some ferrodolomite (shown by the unrecognizable structure, with vitrinite cell-lumens filled with greenish blue by alizarin red combined with potassium resinite seen under microscope; some organic matter shows ferricyanide) and small amount of organic matter (intercry- distinct algous reticulate pattern with clear texture and light stalline) (Fig. 3b). Carbonate crystalline grains and shape are green-yellowish fluorescence, and is considered as excellent poorer, mostly subidiomorphichedral and partially coarse- source rock material[12] (Fig. 3e, 3f). The existence of organic grain, indicating the dolomite were formed in a low energy, laminae indicates an original sedimentary setting featured − quiet and warm setting. Usually in coarse grain, ferrodolomite high biomass productivity, still water and lack of oxygen[13 15]. can be seen replacing calcite and analcite. 2.2. Main assemblage types of fine-grained rocks with Tuffaceous laminae are dark and 10 to 40 μm thick, mostly laminae made up of crystal pyroclast and vitroclastics of silty feldspar (Fig. 3c) with organic matter. The crystal pyroclasts were Above-mentioned different types of laminae can form measured with the maximum extinction angle method of different types of fine-grained rocks alone. Those consisting vertical crystal zone (010) under polarizing microscope, of only one particular kind of laminae are called unitary which shows the plagioclase includes andesine and labradorite sediments, in which the laminae are still recognizable as the (An 30 to 72) of intermediate–basic tuff from volcano minerals within aligned directionally instead of even massive eruption (Fig. 3d). Most crystals are poorly rounded, but there texture. Under fluorescence light, the sediments give off was no volcano erupting around the deposition area, and the alternately dark (dark yellow) and bright (dark green) light. crystal tuffaceous laminae appear alternatively with other However, fine-grained sediments in the study area are mostly tuffaceous laminae, combining all these, it is inferred that they extremely thin lamina couplets (often millimeters in thickness) were formed by materials carried here by monsoon of made up of dolomite lamina with other lamina. Sediments different directions[11], and the chemical deposits of carbonate consisting of two or three kinds of laminae are referred to as in between the laminae might be deposits in still water and binary or ternary lamina couplets, respectively. According to low energy environment during non-monsoon seasons. their different combinations, three types of lamina couplets Organic laminae, also dark under microscope, and 6 to 35 μm have been identified in the study area (Fig. 4, Fig. 5). Type I

Fig. 4. Microscopic features and mineral compositions of major laminae combinations in Lucaogou Formation of Malang sag. (a) and (b): Carbonate laminae (bright) comprising mainly dolomite with well-developed intracrystalline pores and organic/siliciclastic laminae (dark) made up of feldspar debris, quartz, organic matter and small amount of dolomite, Well L1, 3 107.03−3 107.18 m; (c) and(d): Silici- clastic/carbonate lamina couplet, siliciclastic laminae are rich in quartz and contain small amount of feldspar, carbonate (organic matter) laminae contain mainly micrite calcite and dolomite with organic matter filled between crystals, Well L1, 3 190.75−3 190.92 m; (e) and (f) Sample composed of laminae rich in carbonate and laminae rich in organic matter and tuff, the organic matter and tuff laminae are dark and undeveloped in micro-pores, carbonate laminae are dominated by dolomite with well-developed intracrystalline pores, Well L1, 3 143.14−3 413.29 m; (g) and (h): Triple-lamina couplet comprising of carbonate, tuffaceous and siliciclastic laminae. Authigenic quartz can be seen locally. Light-colored siliciclastic laminae are made up of quartz and feldspar debris. Clay minerals are rarely seen. Gray carbonate laminae contain mainly dolomite with well-developed intracrystalline pores. Black tuffaceous laminae contain almost no crystal feldspar pyroclast; Well L1, 3 087.46−3 087.63 m; (a), (c), (e) and (g) are photos taken with polarizers; while (b), (d), (f) and (h) are Qemscan photos of mineral composition of microcell. − 659 − LIU Bo et al. / Petroleum Exploration and Development, 2015, 42(5): 656–666

Fig. 5. Full-scale section photos of lacustrine lamellar fine-grained rocks in Lucaogou Formation of Malang sag. (a) Type I lamina couplet of interbedded siliciclastic and carbonate laminae, with ash black calcilutite with oil stains, oil seepage visible in seams, calcite vein at top, Well N122, 589.78−2 590.00 m; (b) Type II and III lamina couplets, ash black fluorescent calcium-bearing tuffaceous mudstone, Well N122, 2 609.53−2 609.60 m; (c) Type I lamina couplet, ash black calcilutite with oil stains, Well N122, 2 604.52−2 604.78 m; (d) Type I lamina couplet, ash black and oil-stained calcilutite, Well N122, 2 603.88−2 604.13 m. couplets are zebra-striped binary laminae composed of two ment (lagoons or lakes). Extensive algal limestone is found in kinds of laminae (organic/silliciclastic laminae and carbonate southwestern part of Malong sag which mainly contains laminae) or ternary laminae with organic laminae added. Type Collenia. II couplets are also black and white binary combinations of 3. Sedimentogenesis of lacustrine lamellar tuffaceous plus carbonate laminae or ternary laminae with fine-grained rocks organic laminae added. Type III couplets are ternary layers of silliciclastic, carbonate and tuffaceous laminae or quaternary Mixed terrigenous clastic and carbonate sediments are layers with organic laminae added. These fine-grained rocks widespread and complex in deposition mode around the world. comprising multiple layers of different laminae send off much Mount (1985) advanced for the first time the concept of mixed stronger fluorescence when containing organic laminae. sediments[16] and defined it as a mixture of (>10%) terrigenous clastics with (> 25%) carbonate sediments and marl. A 2.3. Organic matter enrichment pattern more generalized meaning of the term include two major Organic matter enriches in the form of laminae, some occur types of sedimentogenesis: high-frequency interaction between in silliciclastic and tuffaceous laminae and a small amount in carbonate sediments and terrigenous sediment input and carbonate laminae. Dark laminae abundant in organic matter mixing of carbonate fraction with terrigenous clastic fraction, have TOC value as high as 8.7%, while light laminae domi- and the latter is a narrowly defined mixed sediments[17]. The nated by carbonate have TOC as low as 0.03%. fine-grained rocks in the study area belong to the former, Maceral analysis of samples from Lucaogou2 Member in which is most commonly seen in mixed terrigenous sedimen- the study area show the maceral includes vitrinite group, tary systems[18]. Paleontological, trace element and biomarker structureless vitrinite group, resintie and amorphous group studies, can be used to reveal their original sedimentary (mineral bituminous matrix), among which amorphous group environments. is the richest (30−60%). Both amorphous group and resinite 3.1. Paleontological characteristics of lamellar give strong yellow-green fluorescence, indicating a low level fine-grained rocks of thermal maturity (low or critical level of maturity) of source rocks. The two groups are good source rocks and have Previous paleontological study of the Lucaogou Formation mostly hydrogen-rich maceral, showing that the organic in Santanghu Basin shows that[19] the formation contains matter type in the study area is sapropelic-prone – organic abundant marine, transition phase, and continental fresh water sludge formed by hydroplankton in stagnant water environ- lake fossils. However, considering the sea-land changes of the

− 660 − LIU Bo et al. / Petroleum Exploration and Development, 2015, 42(5): 656–666 folding systems in northern Tianshan, conditions for lagoons ment rich in H2S of bottom water; B/Ga and Mg/Ca ratios are to be formed near open sea did not exist anymore during the high, reflecting high-salinity of the water body. Moreover, the late Permian and the marine fossils must be “leftovers” of the content of Mn, V, Ba and Sr, and TOC are positively remaining sea water in the area. The marine paleontological correlated, enrichment of these trace elements means high fossils found in the formation are mainly polychaetes with biomass productivity[14], and biomass boom and sedimentation Acerrotrupa (observed to be intergrown with algae) and have a great impact upon the enrichment of trace element in hyolithes. Transition phase fossils are mostly ostracoda and the study area[15]. algae and can be seen almost everywhere in the formation. Biomarkers: Isoprenoid provides important information of Most ostracoda belongs to four species: Tomiella, Darwinula, sedimentary settings of source rocks in the study area (Fig. 7). Iniella and Kemerouiana. Algae are mainly Collenia. Con- Analysis shows that the lamellar fine-grained rocks have tinental fossils are mainly Turpan paleoniscus and few Pr/Ph, Pr/nC17 and Ph/nC18 ratios ranging between 0.15−0.42, phytodetritus (Fig. 6). 0.22−0.38 and 0.25−0.58 respectively, indicating a brackish water - salt lake environment. Regular steranes including 3.2. Geochemical characteristics of lamellar fine-grained ααα20RC sterane ααα20RC sterane and ααα20RC sterane rocks 27 , 28 29 show a “/”-shaped increasing pattern. Despite the fact of

Trace element analysis: Whole rock ICP-MASS (induc- abundant ααα20RC29 sterane, it is inferred that organic matter tively coupled plasma mass spectrometry) analysis shows that in the formation could not be derived from dead higher plants the laminae have V/(V+Ni) ratio higher than 0.78, showing but from algae, most probably blue-green algae[20]. High strong stratification of water body and an anaerobic environ- abundance of hopanes (pentacyclic terpane) and low richness

Fig. 6. Paleontological fossils in samples from fine-grained rocks in Lucaogou Formation of Malang sag. (a) Ancient cod remains in samples from Well M10 at depth of 2 299.60−2 299.80 m; (b) Ancient cod remains in samples from Well M3 at depth of 1 735.80 m; (c) Bioclast limestone from Well M6 at depth of 3 134.20 m, photo taken with cross-polarizers; (d) Algal limestone from Well M6 at depth of 3 134.70 m, photo taken with cross-polarizers.

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Fig. 7. Biomarkers in samples from fine-grained rocks in Lucaogou Formation of Malang sag. of tricyclic terpane imply input of microorganism like bacteria sag is also related to algae flourish, the main evidence is: during the sedimentation of the Formation. Meanwhile high (1) The organic matter mainly comes from algae. Micro- content of γ-cerane indicates high salinity water body or scope observation of organic laminae shows the organic brackish water. Moreover, the richness of β-dancane in sample laminae are amorphous, in the forms of irregular grains, clots extracts from the Formation indicates a reducing–strong re- or floccules. They are lamellar- and stripe-shaped under elec- ducing environment, which is consistent with the low Pr/Ph tron backscattered diffraction microscope. Biomarker analysis ratio. reveals that most amorphisms are formed by biodegradation of phytoplankton. 3.3. Distribution and sedimentation model of different (2) Fine-grained calcite laminae in source rocks are bio-in- laminae duced carbonate sediments. Algae boom in lakes enhanced Statistics of core observation results show that the lamina photosynthesis of water body, exhausting carbon dioxide in structure was controlled by curve and slope of lakeshore of surface water and causing precipitation of calcium carbonate [21] original sedimentary settings (Fig. 8). Shallow water body and into carbonate laminae. its oxygenated environment fostered carbonate laminae (dolo- (3) Enriched and laminated organic matter combined with mite predominated). Deeper water body complicated situation other types of laminae to form rhythmic sedimentary sequen- and formed sedimentary couplets of various colors and shades ces. Algae flourish was a seasonal and periodical phenomenon, of carbonate laminae (tuff and dolomite) with organic and which might result in direct deposition of laminae. Meanwhile, large amount of chlorophyll produced by algae flourish siliciclastic laminae. The darker laminae with high organic caused a reduction of carbon dioxide and a rise of pH value of matter content, often generated hydrocarbon with abundant the salinized lake, which induced precipitation of carbonate γ-cerane. Deep water body gave rise to mainly organic and minerals and formation of dolomite laminae, forming couplets clay-size siliciclastic laminae. Volcanic dust carried by mon- with siliciclastic and carbonate laminae. soon fell and deposited in the paleo-lake, forming couplets of (4) Tuffaceous laminae provide nutrients for the massive tuffaceous laminae with other types of laminae. reproduction of algae and bacteria, thus enhancing biomass 3.4. Mechanisms of organic matter enrichment productivity of paleo-lakes. Analysis of volcanic dust found in the Formation shows that it contains intermediate-basic pla- Algae flourish is considered as the major cause of high gioclase – a frame-shaped and Fe-/Mg-rich aluminosilicate TOC in lacustrine laminae worldwide. from the enrichment of mineral. During volcanic eruptions, the dust carried by wind organic matter in the Permian Lucaogou Formation of Malang dropped into the lake, triggering substantial boom of algae[22]. Moreover, as the dust was fine-grained and had bigger specific area, it possessed strong adsorptivity and sucked in large amount of soluble organic grains, giving rise to tuffaceous laminae with certain amount of organic matter.

3.5. Sedimentogenesis of lacustrine lamellar fine-grained rocks

Sedimentary model of varve has been studied for more than a century in other countries[23]. In terms of genesis, previous work covers ice lake varves, lake varves, marine varves etc. Comparatively speaking, lake varves and lamellar shale are more common than varves of glacier genesis. A consensus view regarding the water body conditions for forming paleo- Fig. 8. Sedimentary model of lamellar fine-grained rocks in lake varves and lamellar shale is that climate factors including Lucaogou Formation of Malang sag. temperature and monsoon are the key elements, and biomass, − 662 − LIU Bo et al. / Petroleum Exploration and Development, 2015, 42(5): 656–666 especially plankton flourish, chemical and biochemical reac- sorbed in the surface of the dusts dissolved, providing large tions as well as mechanical precipitation, are the mechanisms, amounts of nutrients and trace elements for planktons and of the formation of sedimentary laminae[24]. Stratification of bacteria to reproduce. Combined with hot water at the bottom water body is thought to be most favorable for the forming of of the lake, the dissolved materials helped create a favorable laminae[25], as bottom water would suffer from severe anoxia, environment for organisms to flourish. Small amount of sus- which causes death of benthos and prevents textures of sedi- pending terrigenous solids sunk and formed siliciclastic lami- ments from disturbance. Stratified water body also contributes nae. Algae boom destroyed the ecology of the lake and caused a great deal to the preservation of organic matter. Even lami- massive death of organisms, their remains were preserved nae formed during short periods of stratified water body can well by strong reducing environment in the lake. Meanwhile, have TOC value as high as 1.4% to 4.0%[26]. Based on the plankton exhausting carbon dioxide in the water induced car- above discussion, it is concluded the lacustrine lamellar bonate minerals like dolomite to precipitate. In the alka- fine-grained rocks in the study area are controlled by the fol- line-prone water, sulfate reducing bacteria (SRB) reduced 2－ 2+ lowing factors: SO4 into H2S, which then reacted with Fe to form pyrite. 2－ (1) Closed salt lake sedimentary system: the deposition pe- The process made the SO4 content in water drop considera- riod of the Luocaogou Formation saw the largest-scale of bly, leading to the formation of carbonate laminae of powder lacustrine transgression of the Santanghu Basin in the Permian. crystal dolomite. This is how the specific pattern of laminae Trace elements and biomarkers all indicate a high-salinity developed in the Lucaogou Formation of Maglang sag. water body. No trace of terrigenous clastics supply is observed 4. Significance of lamellar fine-grained rocks to and the whole lake basin was relatively closed. The uplift E&P of shale/tight oil and gas in the study area under water is dolomite flat facies consisting of fresh water carbonate spars and dolosiltite. 4.1. High abundance of organic matter (2) Stratified paleo-lake: the formation of laminae is closely The development of organic-rich laminae determines hy- linked to lake stratification. Lack of bentonic fossils, large drocarbon generation capacity of the lamellar fined-grained numbers of well-preserved fossils of fish, abundant pyrite rocks. Overall speaking, the lacustrine lamellar fine-grained grains, inadequate supply of terrigenous clastics as well as rocks in the study area despite strong heterogeneity, are ex- high TOC and biomarker assemblages from geochemical cellent source rocks which have a TOC ranging between 0.5% analysis, all lead to the conclusion that the bottom water was and 18.2%, mostly between 1% and 8%, hydrocarbon genera- anoxia and highly reducing during the formation of the lami- tion potential (S1+S2) higher than 4 mg/g, and chloroform nae, and such bottom water was the result of stratification of bitumen “A” of over 0.1%. In addition, the distribution of lake in the corresponding periods. organic laminae has a positive correlation with hydrocarbon (3) Hot water activities at the bottom of lake: hot water ac- generation potential of the rocks. tivities at bottom of lake affect the growing rate and preservation of organisms by changing markedly sedimentation condi- 4.2. Rich storage space tions including water temperature, oxidation-reduction, and Because of their particular mineral composition and sedi- chemical composition. Silicaclastics and micrite/powder mentary structure, the lamellar fine-grained rocks often con- crystal ankerite possibly linked to bottom hot water activities, tain for a variety of storage space inside. and hydrothermal minerals like analcite, anhydrite, pyrite and (1) Residual pores formed by hydrocarbon generation of etc, are found in laminae in the study area, verifying the con- organic matter. SEM (both SEI and EBSD) images of samples [27] nection . show organic matter residual pores are regular, pit- or honey- (4) Monsoon: Feldspar pyroclast and vitroclastics are found comb-shaped, and dozens to several hundreds of nanometers scattering in the tuffaceous laminae from the study area. Con- in size (Fig. 9a). As residual pores formed after hydrocarbon sidering the fact that there were no homochronous active vol- generation, they are hydrocarbon-wet[28] and vary in size and canoes around the area, the volcanic dust must have gone number from sample to sample because of the heterogeneity through long-distance transportation. The periodical distribu- of organic matter[29]. tion of edged and poor-rounded feldspar grains among organic, (2) Micropores developed in carbonate laminae. Despite the siliciclastic and carbonate laminae indicates that the formation fact that fine-grained rocks have porosity less than 12% in of the tuffaceous laminae is connected with monsoon. During general, the test of samples from the study area show that non-monsoon seasons, the deposits in still water environment carbonate laminae have an average porosity of 18.2% (Well were mostly crystallite carbonate laminae. Niu 122, sampling depth: 2 590 m), and pores with diameter In conclusion, gas and dusts from erupted volcanoes from bigger than 50 μm and averaged at 1 μm are observed in sam- afar were carried by monsoon and fell into the stratified water ples. Free from adsorption, hydrocarbon fluid moves fast in body of the closed salt lake (without terrigenous material sup- pore systems of carbonate laminae than in mudstone[30]. The ply) in the study area. The dust deposited in the lake and pores in carbonate laminae preserved so well may be because formed tuffaceous laminae, meanwhile, aluminosilicate ab- of the acid matters (carbon dioxide, organic acid etc) released

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Fig. 9. Micro-storage space types in lacustrine lamellar fine-grained rocks in Lucaogou Formation of Malang sag. (a) Well-developed organic pores, Well L1, 3 087.46−3 087.63 m; (b) Carbonate laminae mainly composed of dolomite and well-developed intracrystalline pores; (c) Obvious oil stain along bedding, Well L1, 3 088.13−3 088.25 m; (d) Well-developed intracrystalline pores in dolomite with some filled up by authigenic quartz, Well L1, 3 107.03−3 107.18 m. from nearby organic laminae during hydrocarbon generation, shrank and created pores, thus providing space for hydrocar- which entered the relatively closed vertical systems of carbon generated from above and below laminae. bonate laminae, reducing the pH value, thus avoiding the 4.3. Favorable mechanical properties formation of carbonate cement in pores or dissolving the cement if it ever formed[31] (Fig. 9b), and the fact that hydrocar- We know that rock brittleness has something to do with its bon generated within the carbonate laminae could not escape mineral composition. The content of brittle minerals is one of and had to stay inside, boosting up the pressure of the laminae. the key factors affecting the development of fractures in rocks Siliciclastic/tuffaceous laminae have porosity of around 3.6%. and therefore having indirect impact upon oil and gas storage (3) Inter-lamina seams. There are various types of in- and percolation routes[32]. Brittle minerals in siliciclastic and ter-lamina seams in the rocks because: on the one hand, seams carbonate laminae, including quartz, calcite, dolomite and are easily created between laminae under tectonic stress and authigenic quartz formed during conversion of clay minerals connected with high-angle fractures or pores as the laminae (Fig. 9d), can increase the brittleness of the lamellar fine- mostly consist of clay-sized minerals of different mechanical grained rocks, making later fracturing easier. properties and are more than often comprising two types or In conclusion, lamellar fine-grained rocks have good hy- three types of thin laminae of micrometer to millimeter thick- drocarbon generation capacity as they were deposited in en- ness (Fig. 9c); on the other hand, protogenetic calcium car- vironments where algae and bacteria thrived and organic mat- bonate in binary laminae (made up of two types of laminae) ter enriched. Lamellar fine-grained rocks containing micro- 2+ 2－ might react with Mg , CO3 and organic acid and produced pores and fractures (formed and preserved by carbonate lami- a mass of dolomite, during the process, the calcium carbonate nae due to their unstable minerals and weak inter-laminae − 664 − LIU Bo et al. / Petroleum Exploration and Development, 2015, 42(5): 656–666 interfaces) may serve as effective reservoirs for oil and gas. million years ago. SCIENCE CHINA Earth Sciences, 2007, Able to act both as source rock and reservoir, lamellar fine- 37(5): 636–643. grained rocks are ideal self-generation and self-preserving [9] Zeng Qinghui, Qian Ling, Liu Dehan, et al. Organic petro- reservoirs[33]. logical study on hydrocarbon generation and expulsion from 5. Conclusions organic-rich black shale and oil shale. Acta Sedimentologica Sinica, 2006, 24(1): 113–122. Lacustrine lamellar fine-grained rocks have four types of [10] Heydari E, Wade J W, Anderson C L. 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