海陆过渡相页岩储层特征及含气赋存机理——基于与海相页岩储层对比的认识

Reservoir characteristics of marine-continental transitional shale and gas-bearing mechanism:Understanding based on comparison with marine shale reservoir

中国海陆过渡相页岩储层分布广泛,但至今未取得较大的勘探突破。详细调查和分析了海陆过渡相页岩储层特征和含气赋存机理,并展望了海陆过渡相页岩气的研究方向,旨在明确海陆过渡相页岩气成藏机理,以期为有利层段优选提供理论支持。海陆过渡相泥页岩储层具有单层厚度薄、岩性变化快、干酪根类型差等特点。由于有机质纳米级孔数量少,储集空间以黏土矿物孔缝为主。富有机质海相页岩实际解吸气量与理论含气量基本一致,但海陆过渡相页岩实际含气量远低于理论含气量,主要机制为砂泥煤“三明治”空间结构致使大部分天然气高效排出以及高含水饱和度致使游离气储集空间不足。与海相页岩气赋存状态不同,海陆过渡相页岩气以吸附气为主,游离气普遍较低,优势岩相类型为高TOC、高硅质/钙质页岩。海陆过渡相特别是潮坪—潟湖相地层中纵向上发育多层含菱铁矿层、横向上连续分布,在含菱铁矿泥岩内部易形成页岩气“微圈闭”储存气体,且菱铁质泥页岩低孔低渗和高突破压力的特性致使其具有强封闭性,可在页岩内部实现局部超压富集,这为海陆过渡相页岩气垂向有利层段优选和勘探突破提供新的研究视角。今后的研究应加强海陆过渡相系统性沉积学研究,进一步揭示页岩气的赋存状态及动态转化,优化有利层段评价体系以及探索砂泥煤“三气”合采的可行性。

Transitional shale gas layers are widely distributed in China, whereas no significant exploration breakthrough has been made so far. In this paper, characteristics and gas-bearing mechanisms of transitional shale gas reservoirs were investigated and analyzed in detail, aiming to clarify shale gas accumulation mechanism of transitional facies and provide theoretical support for the selection of favorable intervals. Transitional shale gas layer is characterized by thin single layer thickness, rapid lithological change and poor kerogen type. Due to the limited numbers of OM nanometer-scale pores, shale pore space is dominated by pores and fractures related to clay minerals. The measured gas content is well consistent with theoretically calculated gas content for marine organicrich shales. However, actual measured gas content is far lower than the theoretical calculated gas content for transitional shale gas reservoir. The main mechanism are summarized to be that(1) high hydrocarbon expulsion efficiency of “sandwich” space structure of sandstone-shale-coal association gives rise to most natural gas into nearby sandstone, and(2) high water saturation results in insufficient storage space for free gas in shale reservoir. Unlike marine shale gas, natural gas in transitional shale reservoir is primarily dominated by adsorbed gas in kerogen, and free gas is generally low. The favorable lithofacies types are organic-rich siliceous/calcareous shales. Multiple layers of siderite-bearing shale/siderites are developed vertically and are continuously distributed horizontally in transitional strata, particularly in flat-lagoon facies. It is easy to form “micro-trap” to storage gas in siderite-bearing shale, and siderite-bearing shale has strong sealing properties due to low porosity, low permeability and high breakthrough pressure. This property can form overpressure and trap shale gas inside the shale, which provides a new research perspective for the optimization of vertical favorable intervals, as well as exploration breakthrough in transitional shale gas. Further research should strengthen the systematic sedimentological study of transitional facies, reveal shale gas occurrence state and dynamic transformation, optimize favorable interval evaluation system and clarify the feasibility of coal-measure gas commingled production.