准噶尔盆地西北缘中三叠统克拉玛依组烃源岩生烃潜力

Hydrocarbon generation potential of Middle Triassic Karamay formation in the northwestern Junggar Basin

准噶尔盆地西北缘中三叠统克拉玛依组发育有煤系(主要为炭质泥岩和煤)和湖相泥岩两种不同类型的烃源岩.通过TOC,热解,生物标志物和有机岩石学分析后认为克拉玛依组湖相泥岩是一套弱氧化-弱还原条件下的微咸水沉积,而煤系烃源岩则主要是一套弱氧化-弱还原环境下的淡水沉积.研究结果表明:两类烃源岩有机质以陆源高等植物输入为主,同时还有一定比例的藻类和微生物.烃源岩的氢指数平均216 mg/g,H/C原子比平均0.79,有机质类型均为Ⅱ2-Ⅲ型,除生气外,还具有一定的生油能力.克拉玛依组湖相泥岩总体属于中等-好的生油岩,部分达到极好生油岩,丰度高于白碱滩组湖相泥岩;而克拉玛依组煤系烃源岩总体属于中等生油岩.烃源岩镜质体反射率和Tmax均较低(0.41%~0.85%和419~449℃),抽提物甾烷异构体未达到平衡值(C29甾烷20S/(20S+20R)=0.08~0.42;C29甾烷ββ/(ββ+αα)=0.13~0.48),说明总体处于未成熟-低成熟阶段.盆地模拟表明西北缘克拉玛依组成熟有效源灶仅在玛湖凹陷中心部位局限分布,是研究区一套次要的烃源岩.从全盆地来看,克拉玛依组成熟有效源灶的分布范围仍十分可观,有望今后成为准噶尔盆地一个重要的勘探接替领域.

Two types of source rocks were developed in Middle Triassic Karamay formation in the northwestern Junggar Basin,i.e.coaly(primarily carbonaceous mudstone and coal)and lacustrine source rocks.Evidence from geochemistry(TOC,Rock-Eval and biomarker fingerprints)and organic petrology indicated that the Karamay lacustrine mudstone was deposited under sub-oxidizing-sub-reducing brackish environment;while the coaly source rock was deposited under sub-oxidizing-sub-reducing fresh environment.Both of them are dominated by higher plants input with part of alga and microbe and are characterized by kerogen type II2-III with the average hydrogen index and H/C ratio being 216 mg/g and 0.79,respectively,reflecting both oil and gas generation potentials.Most of Karamay lacustrine mudstone was classified as medium-good source rocks and some of them can even reach excellent level.Its quality is generally better than the Upper Triassic Baijiantan lacustrine mudstone.The Karamay coal measures are generally classified as medium source rock.The Karamay source rock has relatively low vitrinite reflectance and Tmax ratios(0.41%-0.85%and 419-449℃,respectively)with its sterane isomers being lower than the equilibrium ratios(C29Sterane20 S/(20 S+20 R)=0.08-0.42;C29Steraneββ/(ββ+αα)=0.13-0.48),indicting immature-low mature stage.It is revealed by basin modelling that the mature source kitchen in the Karamay formation was only confined within the central Mahu sag,covering a small area.Thus,it is less important in the study area.Nevertheless,the mature Karamay source kitchen covers a large area in whole basin scale,making it a favorable target for oil and gas exploration in the future.