川东南—黔北地区龙马溪组沉积环境及对烃源岩的影响

Sedimentary environment and its effect on hydrocarbon source rocks of Longmaxi Formation in southeast Sichuan and northern Guizhou

川东南—黔北地区下志留统龙马溪组发育富含有机质泥页岩,是华南地区海相地层中的优质烃源岩之一,且分布广泛,厚度大,是有利的页岩气勘探区。通过研究区30条野外露头剖面和12口钻井的资料分析,结合岩性、元素地球化学、测井地球物理及古生物等相标志研究,对川东南—黔北地区下志留统龙马溪组沉积环境、沉积演化及其对烃源岩的影响进行了研究,在川东南—黔北地区龙马溪组识别出泥质深水陆棚、泥质浅水陆棚、砂泥质浅水陆棚、砂质浅水陆棚、灰泥质浅水陆棚、灰质浅水陆棚、浊流沉积、台地边缘浅滩-生物礁浊流等8种沉积类型,其中泥质深水陆棚是龙马溪组烃源岩形成的主要沉积环境。通过对沉积环境的研究,认为龙马溪组底部缺氧的滞留环境和缓慢的沉积速率是龙马溪组优质烃源岩发育的主要因素。研究为区内页岩气勘探开发提供有力保障。

Shales in Longmaxi Formation of the Lower Silurian developed in southeast Sichuan and northern Guizhou are rich in organic matters and are high quality hydrocarbon source rocks in marine strata of south China.These shales with great thickness are the source of shale gas and widely distributed in this area.Based on the data of 30field outcrop cross sections and 12wells,the facies marks are summarized in combination with the studies on lithology,element geochemistry,well logging and paleontology.And the sedimentary environment,sedimentological evolution and its effect on the hydrocarbon source rocks are probed into.The eight sedimentary types are identified including deep muddy shelf,turbidite deposit,shallow muddy shelf,shallow sandy and muddy shelf,shallow sandy shelf,shallow marlite and muddy shelf,shallow marlite shelf and reefbank complex on carbonate platform margin.The main sedimentary environment is deep muddy shelf.Among them the deep muddy shelf is the major sedimentary environment for the formation of the hydrocarbon source rocks of the Longmaxi formation.According to the research of the sedimentary environment it is concluded that the anoxic retention environment and the low sedimentary rate in early transgression stage of the Longmaxi sedimentation are the main controlling factors for the development of high quality hydrocarbon source rocks.This study is beneficial to the shale gas exploration of this area.