四川盆地湄潭组生烃潜力分析及勘探意义

四川盆地油气资源丰富,盆地内发育多套富有机质泥页岩,并形成了多套优质生储盖组合。奥陶系是四川盆地下组合目前勘探关注的重要层系,但尚未取得重要油气勘探突破,该成藏组合是否存在烃源岩是该层系油气勘探关注的核心问题之一。下奥陶统湄潭组主要为一套陆棚相泥页岩沉积,分布广、厚度大,其生烃潜力备受关注。基于地层发育特征,通过岩性组合分析、古生物鉴定和微量元素测试,并结合有机质丰度(TOC)参数,结果显示四川盆地在湄潭组沉积期存在川南和川北两个沉积中心:川南沉积中心厚度大,但砂岩或砂质含量多,不利于烃源岩发育;川北沉积中心湄潭组岩性主要为深灰色—灰黑色泥岩、页岩和粉砂质泥岩;疑源类化石和微量元素组成表明沉积环境利于烃源岩发育,TOC达到一般烃源岩标准,因此认为川北地区存在湄潭组烃源岩沉积中心。通过湄潭组有机地球化学分析和源储匹配成藏解剖,结合烃源岩空间分布预测认为,湄潭组烃源岩能为川北—川东北地区上寒武统—奥陶系油气成藏提供烃源保障;以湄潭组为烃源,可形成上生下储、源储通过不整合相接的“湄潭组生—上寒武统洗象池组储”有利成藏组合和下生上储、源储通过断层相接的“湄潭组生—中奥陶统宝塔组储”的潜在成藏组合。湄潭组生烃潜力的确认和2种成藏组合的建立,扩大了川北—川东北地区勘探空间,对上寒武统—奥陶系实现勘探突破具有重要意义。

Hydrocarbon Potential Analysis and Exploration Significance of the Meitan Formation,Sichuan Basin

The Sichuan Basin is rich in oil and gas resources because of the development of multiple sets of rich organic mud shale and combinations of high-quality reservoir and cap rocks. Close attention has been paid to the Ordovician series, but no significant breakthrough has been made in oil and gas exploration. One of the key issues is the existence of source rocks in the Ordovician series. The Lower Ordovician Meitan Formation consists mainly of a set of widely distributed shelf-facies mud shale of large thickness, whose hydrocarbon generation potential has received some attention. Based on the characteristics of formation development, lithological combination analysis, paleontological identification and trace element tests, combined with organic matter abundance parameters(TOC), it is concluded that there were two sedimentary centers in the Sichuan Basin during the sedimentary period of the Meitan Formation. The southern sedimentary center is the thicker, but contains higher sandstone or sandy content which is not conducive to source rock development. In the northern sedimentary center the Meitan Formation is characterized by darkgray to black mudstone, shale and silty mudstone. The presence of acritarch fossils, together with the trace element composition, indicate that the sedimentary environment was conducive to the development of hydrocarbon source rocks, and the TOC is at the standard of source rocks generally. Therefore, it is believed that there is a source rock sedimentary center in the Meitan Formation in the northern Sichuan Basin. Based on an analysis of the organic geochemistry and accumulation anatomy combined with the predicted spatial distribution of the source rocks of the Meitan Formation, it is concluded that they provide a source of hydrocarbon accumulation in the late Cambrian to Ordovician. Overall, in the Meitan Formation it is predicted that a maximum source rock thickness of 40-50 m is located between wells Mashen1, Chuanshen1 and Wutan1. With an average 0.84% TOC and 2.25% Ro, the hydrocarbon generation intensity is calculated to be 22×108 m3/km2, which has the potential to form large and medium-sized oil and gas fields. Two categories of accumulation assemblage are present in the Meitan Formation hydrocarbon source:first, a favorable assemblage where the Meitan Formation source rock is connected to the late Cambrian Xixiangchi reservoir by a planar unconformity, whose anatomy has been confirmed by drilling to be a key element for hydrocarbon accumulation; the second is a potential accumulation assemblage in which the Meitan Formation source rock and the Middle Ordovician fractured reservoir are connected by faults. The Ordovician fractured reservoir is developed in the northeastern Sichuan Basin. The confirmation of the hydrocarbon generation potential of the Meitan Formation and the establishment of two related types of accumulation assemblage has expanded the exploration region in the northern-northeastern Sichuan Basin, which is very significant for an exploration breakthrough of the late Cambrian and Ordovician reservoirs. The tectonic-sedimentary evolution in the northern Sichuan Basin area is likely to have reduced hydrocarbon accumulation in the Meitan Formation on the northern slope of the Chuanzhong Paleozoic uplift and the western slope of the Kaijiang Paleozoic uplift.