黔北地区五峰组—龙马溪组页岩储层含气性特征及其影响因素

Reservoir gas-bearing characteristics and their controlling factors of Wufeng-Longmaxi formations in northern Guizhou

黔北地区上奥陶统五峰组—下志留统龙马溪组发育一套优质富有机质页岩,具有良好的资源前景,但页岩储层特征、含气特征和主控因素尚不明确。基于前人资料和最新钻井,对黔北地区五峰组—龙马溪组页岩的有机质特征、储层物性和含气性特征进行了系统的分析,并探讨了含气性的主要影响因素。结果表明:黔北地区五峰组—龙马溪组为浅水陆棚—深水陆棚沉积,优质页岩主要分布在道真—习水一线深水陆棚相,最大厚度达80~100 m;五峰组和龙马溪组下段富有机质页岩TOC含量高,多在2%以上,有机质类型好、成熟度适中,具有良好的页岩气物质基础;优质页岩脆性矿物含量较高,具有较好的可压裂性,储层空间类型多,以有机质纳米孔为主,孔渗相对较低;页岩的甲烷吸附能力强,主要受控于TOC含量和热成熟度。黔北不同地区页岩含气量差异较大,在习水和道真地区页岩气含量相对较高、在桐梓—正安等地区页岩气含量相对较低,同一构造部位页岩气含量的差异受控于TOC含量、纳米孔隙发育程度和含水饱和度,不同构造部位页岩气含量受控于构造保存条件,构造宽缓、远离大断裂和较大的埋深等保存条件较好的页岩含气量普遍较高。

A set of high-quality organic-rich shale is developed in the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in northern Guizhou,which is expected to have a good resource prospect.However,shale reservoir characteristics,gas-bearing characteristics and their controlling factors of shale gas are still not clear.Based on the previous data and the latest drilling,this paper systematically analyzed organic matter characteristics,reservoir physical property and gas-bearing characteristics of the Wufeng-Longmaxi shales,and further discussed the factors of shale gas content.The results show that Wufeng-Longmaxi formations in northern Guizhou are shallow-deep water shelf facies,and high-quality shales are primarily distributed in the deep-water shelf facies in Daozhen-Xishui areas,with the maximum thickness of 80-100 m.Organic-rich shales in Wufeng Formation and the lower member of Longmaxi Formation have high TOC content,mostly higher than 2%,with good organic matter type and moderated maturity,and therefore they have good conditions for shale gas formation.High-quality shales have high content of brittle minerals,and therefore they are easy to be fractured.Shale pore types are various,dominated with organic matter nano-scale pores,and the studied shales have relatively low porosity and permeability.Methane adsorption capacity of the studied shales is strong,which is mainly controlled by TOC and thermal maturity.Shale gas content in different regions varies greatly.The shale gas content is relatively high in Xishui and Daozhen areas,whereas it is relatively low in Tongzi-Zhengan areas.Gas contents of the shales in the same structural unit is controlled by TOC content,nanometer-sized pore development degree and water saturation,however,shale gas content in different structural unit is determined by structural preservation condition.Shales with good preservation conditions,e.g.,wide and gentle structures,far from large faults and deep burial depth,generally have high gas content.