塔里木盆地东部地区古生界原油裂解气成藏历史分析——以英南2气藏为例

PALEOZOIC OIL CRACKING GAS ACCUMULATION HISTORY FROM EASTERN PART OF THE TARIM BASIN——A CASE STUDY OF THE YN2 GAS RESERVOIR

近年来,在塔里木盆地东部烃源岩高—过成熟区的油气勘探获得重大发现,已在几口井中获得工业气流或油气显示,位于英吉苏凹陷中部英南构造带上的英南2气藏就是其中一个颇具代表性的重要气藏。对其油气的成因和成藏历史的研究引起了地质学家和地球化学家的广泛兴趣。英南2气藏中的天然气属富氮湿气,甲烷和乙烷的稳定碳同位素组成分别在-38.6‰～-36.2‰和-30.9‰～-34.7‰之间。与众多塔里木盆地海相油型气的比较发现,该气藏天然气明显具有成熟度偏高的海相成因的特点。通过对其伴生的凝析油分子标志物的研究可以断定,凝析油不可能来源于浅部的中侏罗统煤系地层,而是与下古生界的寒武系—下奥陶统海相地层密切相关。不过,随之而来的问题是,寒武系—下奥陶统海相烃源岩层目前已经过成熟[Re(等效镜质体反射率)=3%～4%]。从其埋藏史和成熟演化史来看:由于中晚奥陶世的急剧沉降和高古地温,在加里东晚期短短的20Ma(458～438Ma)内,古地温从90℃急剧升高到210℃,以致"生油窗"经历的时间只有10Ma左右,尚未来得及充分排油就已迅速转入生气阶段;现今生油气潜力已很低。再从流体包裹体均一化温度与埋藏—生烃史的数据来看,英南2气藏形成于最近10Ma;而现今寒武系—下奥陶统烃源层不可能直接向气藏提供干燥系数仅0.

Oil and gas exploration in eastern part of the Tarim basin was quite successful recently with several commercial gas accumulations being discoved in high to over-matured source region. Yingnan2 (YN2) gasfield, situated in Yingnan structure of the Yingjisu depression, is one representative gas accumulation. To study oil and gas genetic characteristics and accumulation history in this region attracts wide interests to geologists and geochemists. YN2 gases are rich in nitrogen and relatively wet. The methane and ethane stable carbon isotopic compositions are in the range of -38.6‰^-36.2‰ and (-30.9‰)^-34.7‰, respectively. They are characterized by marine source origin but more matured than variety of other marine originated oil-type gases discovered in the Tarim basin so far. Biomarkers from their associated condensates indicate closely affinity with the Lower Paleozoic Cambrian-Lower Ordovician marine source rocks rather than the shallower Middle Jurassic coal measures. Both gas and associated condensate were obviously derived from the Cambrian-Lower Ordovician marine source rocks in the Yingjisu depression, however, to understand oil and gas generation and accumulation histories remains puzzling. The Cambrian-Lower Ordovician marine source rocks are over-matured (VRE=3%~4%) at present time and their burial and maturity evolution histories indicate that paleo-geotemperature increases rapidly from 90℃ to 210℃ within 20 Ma (458~438 Ma) during the late Caledonian period (Middle-Late Ordovician) due to rapid subsidence and quick heating, which leads source rocks experience very short 'oil window' (about 10 Ma) and incomplete oil expulsion before gas generation stage. They can be regard as 'dead source rocks' with no oil and gas generation potential by the end of Ordovician. On the other hand, fluid inclusion homogenization temperature, burial and hydrocarbon generation histories suggest that YN2 gas in the Jurassic reservoir was formed within recent 10 Ma. It is not possible for the Cambrian-Lower Ordovician source rocks to generate wet gas with dryness coefficient of 0.82~0.90 during this period, whereas the Middle Jurassic strata did not mature enough (R_O< 0.7%) to form such large quantity of oil and gas. Such obvious discrepancy between oil and gas generation, accumulation histories and their properties may reflect special accumulation processes occur in this region. High concentration of diamondoid hydrocarbons detected from condensates may indicate that oil had experienced extensively thermal cracking. Very heavy isotopic composition in whole oil is a supplemental oil cracking evidence. Gas isotope curves illustrate that gas formation temperature is above 190℃. All these evidences suggest that gases discovered in the Tadong area are primarily form by oil cracking, which may turn into the vital exploration target in this region. Oil and gas accumulation processes can be reconstructed as following: the Lower Paleozoic Cambrian-Lower Ordovician marine source rocks experienced oil peak-wet gas-kerogen cracking dry gas stages during the late Caledonian movement, oil and gas migrated upward along faults to the Middle-Upper Ordovician or Silurian sandstone reservoirs to form ancient oil and gas accumulations. With increasing burial depth during the late Yanshanian movement, these deep buried ancient oil accumulations start to cracking and large quantity of cracked gas migrates upward. Since incomplete trap development and poor tightness of top seal at this time, methane and other light components were preferentially diffusion and leaking away, leading obvious compositional fractionation in gas accumulation process and concentrated wet gas components. With trap tightness getting better and continuous gas charging in the late Himalayan movement, gases filling into the traps and leakage away through faults reach a dynamic status in certain scale, which finally forms deep sourced secondary condensate accumulations in the study area.