川西中二叠统栖霞组岩石学特征及沉积相研究

Lithological Characteristics and Sedimentary Facies of Middle Permian Qixia Formation in Western Sichuan Basin

摘要: 近期四川盆地二叠系栖霞组获得多项油气勘探突破，如川西北双鱼石区块已发现大型油气藏。然而在实际勘探开发过程中，栖霞组储层段在不同钻井上的气测显示差别很大，且部分井发生失利的情况，说明储层具有很强的非均质性，沉积相作为优质储层的关键控制因素尚需深入研究。针对这一问题，本次研究通过详实的野外露头和大量的薄片鉴定分析，系统梳理了栖霞组不同类型的岩石特征和沉积相类型。本次研究认为，川西地区栖霞组碳酸盐岩按照岩石形态可划分为四种岩石类型：层状灰岩、云斑灰岩、砂糖状白云岩、生物礁灰岩。根据结构分类方案，川西地区栖霞组碳酸盐岩可分为石灰岩和白云岩，其中石灰岩可细分为灰泥石灰岩类、颗粒石灰岩类、礁石灰岩和钙华石灰岩，白云岩可细分为泥粉晶白云岩、细晶白云岩、中粗晶白云岩和白云石胶结物。川西栖霞组主要为一套的弱镶边台地相沉积，包括滩、礁、台洼三种亚相。栖霞组滩相以薄–中层灰泥–亮晶颗粒滩和灰泥颗粒滩为主，在台缘和台内均有发育，高能滩较为少见，以中低能滩为主，且滩体不是连片发育，非均质性较强。台洼是栖霞组最为主要的沉积相类型，其中开阔台洼分布范围最广，岩性以生屑质灰泥石灰岩为主。因滩体分隔使得水体相对局限，Mg/Ca比值升高，使其具有发生中等盐度回流渗透白云岩化作用的条件，从而促进了局限台洼和周缘滩体中的石灰岩发生白云岩化而形成优质储层。

Abstract: Recently, a number of breakthroughs in oil and gas exploration have been made in the Permian Qixia Formation in the Sichuan Basin, such as the discovery of large oil and gas reservoirs in Shuangyushi area in the northwest Sichuan Basin. However, during the actual exploration and development process, different well test production of Qixia Formation reservoirs varied greatly, and some wells experienced failures, indicating that the reservoir has strong heterogeneity and the sedimentary facies as a key control factor for high-quality reservoir needs to be further studied. To address this issue, this study systematically sorted out the different types of rock features and sedimentary faces of the Qixia Formation through detailed field outcrops and extensive thin section identification analysis. In this study, the carbonates of the Qixia Formation in the western Sichuan area can be classified into four rock types according to their rock morphology: laminated limestone, porphyritic limestone, sugary dolomites, and reef limestone. According to the structural classification scheme, the carbonate rocks of Qixia Formation in western Sichuan can be divided into limestone and dolomite, of which the limestone can be subdivided into lime mudstone group, grain limestone group, reefstone group and calcareous limestone, and the dolomite can be subdivided into micrite dolomite, fine crystalline dolomite, medium coarse crystalline dolomite and dolomite cement. The Qixia Formation in western Sichuan is mainly composed of a set of weakly bound platform facies, including three subfacies of shoal, reef and platform sag. The beach facies of Qixia Formation is dominated by thin to medium limudy-sparry grain beaches and limudy grain beaches, which are developed both at the platform margin and within the platform. High-energy beaches are rare, with medium to low-energy beaches predominating, and the beaches are not contiguous and have strong heterogeneity. The platform sag is the most important sedimentary facies type of Qixia Formation, among which open platform sag is the most widely distributed, and lithology is dominated by bioclastic lime mudstone. The relatively restricted water bodies and elevated Mg/Ca ratios due to the separation of the beaches provide conditions for medium salinity reflux seepage dolomitization, which promotes dolomitization of limestone in restricted platform sag and surrounding beaches to form high quality reservoirs.