西藏措勤盆地上侏罗统萨波直不勒组的发现及其烃源岩

措勤盆地东部班戈县保吉乡各昌茶玉地区发现晚侏罗世珊瑚化石约10属15种及未定种,分别为Actinaraea?sp.,Allocoenia?sp.,A.matheyi(Koby),Clausastraea?sp.,Hydnophora?sp.,Latiastraea minima(Koby),Latiastraea?sp.,Meandraraea?sp.,Stylosmilia crassitheca Liao et al Li,S.corallina Koby,Thamnasteria?sp.,T.concinna(Goldfuss),Thecosmilia.baojiensis sp.nov.,Thecosmilia?sp.,Trocharaea cf.patelliformis Gregory等。化石指示各昌茶玉剖面第1—41层地层时代为晚侏罗世牛津期—基默里奇期。珊瑚化石产出的层位原来被认为是混杂在中侏罗世蛇绿岩中的中二叠统下拉组,本文根据剖面中化石及岩性组合特点将其修订为上侏罗统萨波直不勒组,并得出了该地区不存在原来所认为的中二叠统下拉组与中侏罗统蛇绿岩构造混杂现象的认识。本次萨波直不勒组与改则县物玛乡抢古村建组剖面相对比,其岩性更细、灰岩夹层更多,珊瑚化石更为丰富,代表了更远离陆源碎屑源区的沉积环境,进一步证明晚侏罗世冈底斯地区地势是从北向南逐渐加深的,措勤盆地在晚侏罗世处于活动型深海-半深海相和上斜坡相的沉积环境中。保吉地区萨波直不勒组顶部存在上斜坡相68 m厚黑色泥页岩,TOC(总有机碳)平均值为0.83%,属于中等-好烃源岩,剖面第20层12 m厚黑色泥岩TOC值为0.65%,属于中等烃源岩。萨波直不勒组顶部为上斜坡相沉积,分布范围应该比上覆的吐卡日组更广泛。经计算,措勤盆地内萨波直不勒组烃源岩的生烃量约为44.373亿吨。萨波直不勒组的泥页岩为措勤盆地提供了充足的油源保障,同时其中的碳酸盐岩夹层也可能是油气有利储层,建议对上侏罗统萨波直不勒组开展钻探取样工作,获取更为翔实丰富的有机地球化学参数。

The Discovery of the Upper Jurassic Sabozhibule Formation in Coqen Basin, Tibet, and Its Source Rock

About 10 genera and 15 species or undefined species of coral fossils are recognized in the Gechangchayu section, eastern Coqen Basin, Baoji area, Bange County, Tibet. They are Actinaraea? sp., Allocoenia matheyi (Koby), Allocoenia? sp., Clausastraea? sp., Hydnophora? sp., Latiastraea minima (Koby), Latiastraea? sp., Meandraraea? sp., Stylosmilia crassitheca Liao et al Li, S. corallina Koby, Thamnasteria? sp., T. concinna (Goldfuss), Thecosmilia? sp., T. baojiensis sp. nov., Trocharaea cf. patelliformis Gregory. These corals indicate the age of related strata is Late Jurassic Oxfordian-Kimmeridgian. This section, which was used to be considered as middle Permian limestone blocks of Xiala Formation tectonically mixed in the middle Jurassic ophiolite complex, was redefined to Sabozhibule Formation based on both lithostratigraphic and biostratigraphic comparison in this study. The authors hold that there is no tectonic complex phenomenon happening among the Middle Permian Xiala Formation and the Middle Jurassic Ophiolite in this area. In addition, Gechangchayu section should be located far away from terrigenous detrital source area according to the fine-grained rock, more limestone interlayers and more abundant corals in comparison with the stratigraphic construction section in Qianggu Village, Wuma Town, Gaize County. All the evidence further proves that the sedimentary environment of Gangdese area gradually deepened from north to south during the Late Jurassic, the Coqen Basin was situated at an active pelagic-hemipelagic and slope facies sedimentary environment during the early time of Late Jurassic. There are 68m thick shales on the top of the Sabozhibule Formation, and the average TOC value is 0.83%, which indicates that they are medium- good source rocks. The TOC value of mudstone with the thickness of about 12 m at the 20th layer of Gechangchayu section is 0.65%, which belongs to the medium source rock. The top stratum is in the upper-slope facies, therefore its distribution range should be wider than the Tukari Formation. The calculated hydrocarbon generation quantity of source rocks in the Sabozhibule Formation is about 44.373×108 t within the whole Coqen Basin. Shale in the Sabozhibule Formation provides sufficient oil source for the Coqen basin, and the carbonate intercalations are also favorable reservoirs. It is suggested that sampling should be carried out in the Sabozhibule Formation to obtain more detailed and abundant organic geochemistry parameters.