琼东南盆地基底花岗岩风化—构造联合控储机制

Combined control mechanism of weathering and tectonics for basement granite of Qiongdongnan Basin,South China Sea

含油气盆地的潜山基岩油气藏是一个特殊的勘探领域。近年来,在南海琼东南盆地的中生界花岗岩潜山中获得多个油气勘探发现,昭示着该领域良好的勘探前景。盆地中部的松南潜山群资料最为丰富,多口探井钻遇基底花岗岩储层。裂缝作用作为致密结晶基岩成储的先决改造条件,在基岩潜山储层研究中至关重要,但是对该区域花岗岩潜山的裂缝发育特征以及构造应力缺乏研究,严重制约了基岩潜山油气藏的勘探进展。基于松南低凸起5口钻井的岩心和壁心观察、显微薄片鉴定、测井和地震资料解释,揭示了松南低凸起潜山的多期裂缝的切割关系、开度、形态和走向等发育特征,并探讨了形成两组构造缝的构造应力。研究结果表明,构造缝和沿裂缝的溶蚀孔洞是基岩花岗岩的主要储集空间;构造缝分为北东向和北西向两组,印支期古特提斯洋关闭形成的北东向挤压是北西向断裂以及北东向构造缝的构造动力,燕山期来自东南侧的西太平洋俯冲相关的构造挤压则促成了北东向断裂以及北西向构造缝的发育;花岗岩中高密度裂缝的发育为暴露期大气水下渗溶蚀提供了网状通道,并形成了由砂砾岩带、风化裂缝带、内幕裂缝带组成的风化壳储层三层结构;新生代早期裂谷盆地形成过程中的伸展拆离背景对先存网状裂缝体系具有活化作用,是裂缝松弛开启从而提高储集空间的重要因素。

Reservoirs within buried hills in petroliferous basins represent a unique area of exploration.Recent discoveries in the Mesozoic granite buried hills of the Qiongdongnan Basin in the South China Sea show promising exploration prospects.Among these,the Songnan buried hill group in the basin’s central part has rich drilling data,with several wells encountering basal granite reservoirs.Fracturing is crucial for reservoir formation in dense crystalline bedrock,but limited research on fracture characteristics and tectonic stress hampers basement buried hill reservoir exploration.This study examines multi-stage fracture development in the Songnan low uplift buried hill based on core observations,thin-section analysis,well logging,and seismic data from five wells.It analyzes cutting relationships,openness,morphology,orientation,and explores tectonic stresses forming two groups of fractures.Tectonic fractures and dissolution holes along them are the main reservoir spaces within the granite basement.They are categorized into northeast(NE)and northwest(NW)trending families.NE trending frac-tures result from the Paleo-Tethys Ocean closure during the Indosinian period,influencing NW trending fractures.Tectonic extrusion related to western Pacific Ocean subduction during the Yanshanian period contributes to both NE and NW trending fractures.High-density fractures in granite allow atmospheric and underwater seepage and dissolution,forming a three-layer weathered crust reservoir structure:sandstone conglomerate belts,weathering fracture zones,and inner fracture zones.The extensional detachment background during early Cenozoic rift basin formation activates pre-existing fracture systems,enhancing reservoir space through fracture relaxation.