Characteristics of paleokarst processes and reservoir development of the middle Permian Maokou Formation in the Yuanba area,Sichuan Basin

JI Shaocong; ZHANG Qingyu; BA Junjie; NIE Guoquan; DONG Hongqi; MO Guochen; ZHANG Meng

doi:10.11932/karst20250215

Abstract：In recent years,the Maokou Formation of the middle Permian in the Yuanba area of Northwest Sichuan has consistently yielded high-flow industrial gas,indicating significant exploration potential for the Maokou Formation in this area.Drilling has confirmed the presence of exposed karst features within the Maokou Formation,indicating that the reservoir space mainly consists of fracture-pore types.The karst fracture-pore reservoir type is one of the important reservoir types found in the Maokou Formation in this area.Previous studies have been conducted on the stratigraphic division,lithofacies paleogeography,karst layer groups,and karst paleogeomorphology of the Maokou Formation in Northwest Sichuan.However,systematic research on paleokarst stages of the Maokou Formation in the Yuanba Area,as well as its impacts and other related aspects,has been limited.This study investigates the development characteristics and distribution patterns of karst reservoirs in the Maokou Formation of the Yuanba area in the Sichuan Basin,based on a study on the geological background of ancient rock dissolution.The investigation utilizes field profiles,drilling cores,logging,and analytical testing data.This study combines geochemical analyses of the filling materials found in paleokarst fractures and pores,to explore the stages of ancient rock dissolution and its impacts on karst reservoirs.The findings will support predictions regarding the karst reservoirs in the Maokou Formation in the Yuanba area in subsequent research. The research results indicate,(1)Taking the top surface of a section in the Maokou Formation as the underlying reference surface,the residual thickness method was used to restore the karst paleogeomorphology of the top surface of the Maokou Formation in the study area.The karst paleogeomorphology of the top surface was categorized into four secondary geomorphological units and six tertiary geomorphological units.Overall,the karst paleogeomorphology on the top surface of the Maokou Formation in the study area is characterized by micro-geomorphological features,with locally developed mature landforms that indicate the initial stages of karst formation and evolution.The surface drainage system is not fully developed and is influenced by the topography,with runoff flowing northward and southwestward from the central region.(2)Based on the results of field profiles,drilling core observations,thin section identification,and scanning electron microscopy observations,the rock types of reservoirs in the Maokou Formation within the study area are mainly mud-crystal bioclastic limestone,bright-crystal bioclastic limestone,and a minor amount of bioclastic mud-crystal limestone.The types of storage space are classified into three categories:dissolved pores,caves,and fractures.(3)According to the classification guidance formulated by the Institute of Karst Geology,Chinese Academy of Geological Sciences,karst reservoirs in the Maokou Formation are vertically classified into four karst development zones,that is,surface karst zone,vertical infiltration and dissolution zone,runoff dissolution zone,and phreatic dissolution zone.This classification was based on the observation of field profiles,drilling core samples,logging,and the analysis of well logging and drilling records in the study area.The vertical distribution of karst reservoirs in the Maokou Formation is primarily concentrated within 0-20 m below the weathering crust surface,corresponding to the surface karst zone.(4)The analysis of carbon and oxygen isotopes,along with inclusion testing of the filling materials in the paleokarst fracture-pore of the Maokou Formation in the study area indicates that the paleokarst fracture-pore system of the Maokou Formation was formed in four distinct karst environments,a contemporaneous or pene-contemporaneous karst environment,a freshwater karst environment in the hypergene period,a karst environment in the shallow-burial period,and a high-temperature karst environment in the deep-burial period.(5)Comprehensive analysis suggests that favorable sedimentary facies such as open platforms or platform edges are the material basis for the development of karst reservoirs in the Maokou Formation in the study area.Epigenetic karstification is a key factor in the development of karst reservoirs in the Maokou Formation in the study area,and its degree of development is controlled by conditions such as the karst layer group,ancient landforms,and ancient water systems.The shallow-deep-burial process has transformed and adjusted the early reservoir space,further increasing the heterogeneity of the reservoir.