川西地区雷口坡组沉积与其顶部风化壳储层特征

Characterization of the sedimentation of the Leikoupo Formation and the weathering crust reservoirs at the top of the formation in the western Sichuan Basin

四川海域在印支晚拉丁末期隆升前,中三叠统雷口坡组主要为陆表海断续淹没的蒸发台地,台地整体呈东高西低的古地理格局,台内发育的次级隆凹是蒸发台地上潟湖、咸化盐湖的基底。雷三—雷四段沉积时期,咸化湖盆低凹处随东高西低格局的演化迁移到川中-川西的南充、成都一带;拉丁期末的"印支早期运动"使四川海域整体隆升,在雷口坡组顶部形成分布广泛的古风化壳。川西地区雷口坡组顶部古风化壳主要形成于雷四段的上部,风化壳主要经历了暴露溶蚀和深埋溶蚀两大阶段的成岩作用,在风化壳内形成与成岩作用密切相关的缝洞型储层。缝洞型储层发育与否是雷口坡组顶部成藏的主控因素之一。川科1井已在川西坳陷孝泉构造雷口坡组顶部获得天然气勘探重大突破。通过对风化壳成因及控制储层发育的主控因素分析,指出川西地区雷口坡组顶部的勘探潜力巨大。

Before the uplifting in the late Ladinian age of the Indosinian, the Leikoupo Formation of the Middle Triassic in Sichuan Basin was an evaporite platform intermittently submerged by epicontinental sea. The overall paleogeographic pattern of the platform was high in the east and low in the west. The secondary uplifts and de- pressions developed in the platform acted as the basement for lagoon and salinized salt lake. The depocenter of the salt lake moved with the evolution of the paleogeographic pattern to areas along Chengdu and Nanchong in central and western Sichuan Basin during the deposition of the third and fourth members of the Formation. The Sichuan sea was uplifted as a whole due to the early Indosinian movement in late ladinian age, leading to the de- velopment of a wide-spreading paleo-weathering crust at the top of the Formation. In western Sichuan Basin, the ernst is mainly located at the top of the fourth member. Exposed dissolution and deep burial dissolution led to the development of fractured-vuggy reservoirs within the crust. It is observed that the development of vuggy- fractured reservoirs is one of the major factors controlling hydrocarbon accumulation. A big breakthrough of gas exploration has been made with the well Chuanke - 1 at the top of the Formation in the Xiaoquan structure of western Sichuan Basin. The development shows that the analysis of the genesis of weathering crust and the main controlling factors for reservoir top of the Formation is the potential exploration targets in western Sichuan Basin.