Applciation of UVA oblique photogrammetry in multi-scale characterization of Yunhuatan shoal reservoir in the Maokou Formation,central Sichuan Basin:A case study on the Hongyan outcrop section,Huaying

CHEN Cong; DAN Yong; CHI Yuchao; CAO Hua; DENG Min; LI Changwei; YUAN Haifeng; CHENG Jinxiang; WAN Lijun; CHEN Gongyang

doi:10.11932/karst2025y028

Abstract：In recent years,guided by the concept of facies-controlled reservoirs,high-quality porous dolomite reservoirs of platform margin facies in Member 2 of the Maokou Formation(Mao-2 Member)have been discovered on the northern slope of Central Sichuan and in the Moxi-Longnüsi-Hechuan-Tongnan area,achieving significant exploration breakthroughs.However,traditional carbonate reservoir characterization is limited by the scale and scope of drilling boreholes.Direct observation at the macro-scale is difficult;multi-scale characterization of the reservoir is even more challenging,and understanding the distribution and development patterns of the reservoir remains elusive.This study employs multiple novel technologies,utilizing macroscopic field outcrops as the research subject,to conduct multi-scale characterization of the dolomite reservoir in the Maokou Formation at the Hongyan Outcrop Section of Huaying Mountain in Central Sichuan.Using UAV oblique photogrammetry and the EPS 3D mapping software system for data processing,a 1.2 km full-section real-scene model of the Maokou Formation was constructed at the macro-scale,with imagery precisely matched to geographic coordinates.For the key reservoir interval of the Yunhuatan Shoal in the lower sub-member of Mao-2(Mao-2L),medium-scale,high-precision imagery covering approximately 100 meters was acquired via UAV wall-hugging flight,achieving centimeter-level recognition accuracy.Combined with continuous drilling samples,cast thin sections,porosity-permeability tests,and previous research results,the reservoir petrology,pore structure,and petrophysical characteristics of the Yunhuatan Shoal Reservoir interval were systematically studied at the micro-scale.Finally,by integrating macro-(full-section model),meso-(reservoir interval imagery),and micro-scale(thin sections and experiments)data,a 3D parametric model of the Yunhuatan Shoal Reservoir was constructed,enabling the integrated multi-scale characterization of reservoir characteristics.By merging traditional methods with digital technology,this study establishes a"macro-meso-micro"multi-scale research methodology,providing a novel approach for carbonate reservoir characterization. The results indicate as follows,(1)The section comprises the development of two third-order sequences.The Yunhuatan Shoal Reservoir is located in the middle-upper part of Mao-2L,deposited in a shallow environment on the margin of a carbonate platform during the highstand systems tract of SQ1.Reservoir rocks are classified into three types:pure dolomite,transitional lithology dolomite(dolomite with limestone patches or dolomitic limestone),and limestone.Fine-crystalline dolomite within pure dolomite is controlled by the beach core microfacies.Dolomite with limestone patches(dolomitic limestone)and limestone are developed in the beach flank facies,while limestone is controlled by the interbeach microfacies.Sea-level fluctuations controlled the development of two stages of Yunhuatan Shoal.(2)Multiscale reservoir characterization shows that the reservoir space can be classified into three types:(a)Dolomitization-related intercrystalline pores,significantly controlled by facies;(b)Tectonic fractures,related to multi-stage tectonic activity;(c)Dissolution-related pores,intercrystalline dissolution pores,dissolution vugs along fractures,and calcite vugs indicating at least two discrete phases of fluid flow(karst)events,in addition to the current karst-related pores/vugs that has been excluded from this study.(3)Porosity-permeability analysis indicates the following hierarchy:beach core facies＞beach flank facies＞interbeach facies.Higher proportions of dolomite patches,together with enhanced connectivity,corresponds to improved reservoir quality.Dolomitic patches hosted in limestone exhibit superior porosity and permeability compared to dolomite patches within dolostone.However,when the dolomite content falls below 50％,the porosity and permeability of patchy dolomitic limestone approach those of bioclastic micritic limestone and pure micritic limestone,indicating that dolomitization does not confer a significant petrophysical advantage in such lithofacies. In summary,through multi-scale characterization,a developmental model of the dolomite reservoir in the Mao-2 was successfully established,providing a foundation for subsequent reservoir evaluation and research into its formation mechanisms.