Mechanism of organic-rich shale formation and shale gas enrichment in the Carboniferous Tian’eping Formation from the Xiangzhong Depression
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摘要:研究目的
本文旨在通过页岩成因和页岩气富集机理研究，确定湘中坳陷下石炭统天鹅坪组页岩气形成富集主控因素和富集模式。
研究方法以湘新地4井为重点，通过下石炭统碳酸盐岩稳定碳、氧同位素、页岩全岩氧化物和微量元素含量测定，分析页岩形成的古气候、古环境特点，确定富有机质页岩的成因。在系统查明页岩岩石矿物学、有机地化和储存物性特征以及页岩气赋存方式和构造保存条件基础上，结合中—低成熟页岩储存的热演化模拟结果，确定页岩气富集机理。
研究结果（1）下石炭统天鹅坪组富有机质页岩是全球早石炭世杜内期气候剧烈波动引起海水分层，海底缺氧的沉积产物。（2）涟源地区中生代广泛而强烈的岩浆事件导致区域古地温梯度升高，并引起天鹅坪组页岩发生二次生烃和储层物性的改善。天鹅坪组页岩气是印支期油气调整后，页岩中原油裂解和有机质二次生烃的共同结果。（3）发育在下石炭统测水煤系中的滑脱构造部分封堵了下伏天鹅坪组页岩气的垂直逸散通道，有利于页岩气的保存富集。
结论湘中坳陷下石炭统天鹅坪组页岩气是有利相带控制总有机碳含量、岩浆热作用控制储层物性和滑脱构造控制保存的共同结果。
创新点:（1）气候重大转折期古大陆边缘凹陷盆地是页岩气勘探的有利相带；（2）岩浆热作用制约区域页岩有机质热演化程度和页岩气储存的品质；（3）滑脱构造下盘是页岩气勘探的最有利区域。
Abstract:This paper is the result of oil and gas exploration engineering.
ObjectiveThrough the study of shale formation and shale gas enrichment mechanism, the current study aims at finding out main factor controlling shale gas reservoir in the Carboniferous Tian’eping Formation in the Xiangzhong Depression as well as its enrichment patterns.
MethodsCarbonates carbon and oxygen isotopes as well as shale trace elements and major elements were analyzed at Xiangxindi 4 core in order to recover paleo−environment and investigate the origin for the shale formation. Combined with the thermal evolution simulation of medium−low maturity shale gas reservoirs, the mechanism for shale gas enrichment is identified through petromineralogy, organic geochemistry, physical properties of shale gas reservoirs, existence forms of shale gas and tectonic preservation condition.
Results(1) The organic−rich shale in Lower Carboniferous Tian’eping Formation were formed due to seawater stratification and seabed anoxia caused by the intensive climatic fluctuations in the Early Carboniferous. (2) The extensive and intense magmatic events in central Hunan led to the increase of locally paleogeothermal gradient and further caused secondary hydrocarbon generation in the Lower Carboniferous organic−rich shale. Shale gas in the Tian’eping Formation was formed owing to crude oil cracking and secondary hydrocarbon generation of organic matter. (3) Shale gas preservation was promoted due to decollement in the Ceshi Formation of the lower Carboniferous blocking the vertical escaping channel of the shale gas from the underlying Tian’eping Formation.
ConclusionsThe shale gas of Lower Carboniferous Tian’eping Formation in central Hunan Depression is the common result of favorable facies zone controlling the total organic carbon content, magmatic thermogenesis controlling the reservoir physical properties and detachment structure controlling the preservation.
Highlights:(1) Paleo depression in the continental margin during the major climate transition period is benefit for shale gas exploration. (2) Organic matter thermal evolution in regional shale and shale gas preservation are limited by magmatism. (3) The decollement footwall is the most favorable area for shale gas exploration.
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图 1 湘中坳陷地质简图（a）和页岩气勘探程度图（b）
Figure 1. Geological sketch (a) and shale gas exploration degree map (b) of the Xiangzhong Depression
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图 2 湘新地4井下石炭统岩性、伽马、总有机碳，δ13C、含水饱和度与气测值
Figure 2. Lithology, gamma, TOC, δ13C, water saturation and gas content of the Lower Carboniferous in Xiangxindi 4 Well
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图 4 湘中下石炭统富有机质页岩形成的古地理、古环境与古气候指标
a—岩相古地理图；b—沉积相模式示意图；c—Zr/Ti–CIA相关性图；d—Zr/Ti–V/(V+Ni)相关性图
Figure 4. Palaeogeography, palaeoenvironment and palaeoclimate index of the formation of organic-rich shale in the Lower Carboniferous in Central Hunan Province
a–Lithofacies paleogeographic map; b–Schematic diagram of sedimentary facies model; c–Correlogram of Zr/Ti–CIA; d–Correlogram of Zr/Ti–V/(V+Ni)
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图 5 湘中下石炭统物源、盐度，气候和生产力指标变化曲线
Figure 5. Provenance, salinity, climate and productivity index of the Lower Carboniferous in Central Hunan Province
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图 6 不同模拟温度下页岩储层的物性和含气性特点
a—C7−1的形貌变化：a1—原始样品，a2—300℃，a3—500℃；b—C7−1有机质分布特点：b1—350℃有机质与矿物颗粒间孔，b2—400℃黄铁矿晶体间有机质，b3—450℃有机孔；c—C7−1沥青反射率；d—C7−1组孔体积分布图；e—孔隙度分布图；f—渗透率分布图；g—总孔体积对比图；h—微孔体积对比图；i—C7−2组甲烷等温吸附曲线图；j—C7−3组甲烷等温吸附曲线图
Figure 6. Physical and gas properties of shale reservoirs at different simulated temperatures
a–Morphologic change of sample C7−1: a1–Primitive sample, a2–300℃, a3–500℃; b–Distribution of the organic matter in sample C7−1: b1–The pore between the organic matter and mineral grain at 350℃; b2–The pyrite intergranular pores filled with organic matter at 400℃, b3–Organic pore at 450℃; c–Bitumen reflectance of sample C7−1; d–Pore volume of sample C7−1; e–Porosity; f–Permeability; g–The total pore volume; h–Micropore volume; i–Isothermal adsorption curve of methane of sample C7−2; j–Isothermal adsorption curve of methane of sample C7−3
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图 7 湘新地4井下石炭统页岩气产出曲线
Figure 7. Shale gas production curve of the Lower Carboniferous in Xiangxindi 4 Well
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图 8 湖南涟源下石炭统页岩气双冲断裂夹块控藏模式图
F1—测水组内部顺层逆冲断裂；F2—泥盆系底部逆冲断裂带；C—测水组底界；D, L—二维地震测线
Figure 8. Controlling reservoir model of the Lower Carboniferous shale gas in Lianyuan, Hunan Province
F1–Bedding thrust fault within the Ceshui Formation; F2–Thrust fault at the base of the Devonian; C–The base of the Ceshui Formation; D, L–2D seismic line
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