Abstract

Weathered-crust karst carbonate is developed in the third submember of the fourth member of the Middle Triassic Leikoupo Formation (Lei43 submember) in the Longgang area of Sichuan Basin, China, and acts as an important oil and gas reservoir. To reconstruct the paleogeomorphology of the weathered-crust karst, we analyzed the seismic thickness of the interval from the erosional unconformity surface on the top of the Leikoupo Formation to the karst at the bottom of the third member of the Xujiahe Formation (Xu3 member), based on existing three-dimensional seismic and drilling data, and the sedimentary characteristics of the Leikoupo and Xujiahe formations. There are three secondary types of geomorphological unit, namely the karst highlands, the karst transitional zone, and the karst basin, and these have distinct karst water development patterns and hydrological conditions that determine the intensity of karstification and reservoir quality. Among them, the karst highlands have a poor reservoir capacity due to their long-term exposure above the water table, which resulted in severe denudation. The karst transitional zone features the superimposition of multiple periods of strong karstification, and this has resulted in high-quality reservoir conditions in the karst monadnocks but poor reservoir conditions in the karren. The karst basin represents a drainage area that experienced weak karstification, and the reservoir capacity is generally poor, although some good reservoirs were developed in shoals. Paleogeomorphological maps provide excellent guides to finding karst monadnocks in karst transitional zones, and monadnocks should serve as the main targets of exploration in the weathered-crust karst reservoir on top of the Lei43 submember.

Highlights

  • IntroductionIt is widely known that 60% of the total oil and gas reserves worldwide are in carbonate reservoirs (Cao et al, 2010, 2015; Guo and Zeng, 2015; Huang et al, 2015; Morad et al, 2016; Roehl and Choquette, 1985; Shi et al, 2017; Tao et al, 2016; Wang et al, 2015; Wei et al, 2017; Yang et al, 2017), among which weathered-crust karst reservoirs are an important type (Feazel, 2010)

  • Li (2001) proposed that shoals were the foundation for reservoir development, but Qin and Zou (2012) argued that reservoir quality is controlled by the superimposition of weathered-crust and sedimentary facies, and Zhong et al (2011) suggested that weathered-crust karst is the key for reservoir formation

  • Recent research on the Leikoupo Formation in the Longgang area has shown that intra-platform shoals form the basis of reservoirs, while their distribution is related to karst drainage controlled by paleokarstification (Ding et al, 2012; Li et al, 2011; Qin and Zou, 2012; Zhong et al, 2011)

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Summary

Introduction

It is widely known that 60% of the total oil and gas reserves worldwide are in carbonate reservoirs (Cao et al, 2010, 2015; Guo and Zeng, 2015; Huang et al, 2015; Morad et al, 2016; Roehl and Choquette, 1985; Shi et al, 2017; Tao et al, 2016; Wang et al, 2015; Wei et al, 2017; Yang et al, 2017), among which weathered-crust karst reservoirs are an important type (Feazel, 2010). Much detailed research has been conducted on the main factors involved in the genesis and the patterns of the weathered-crust reservoir, while little has been conducted on reconstructions of the karst paleogeomorphology, which means there has been little effective guidance for hydrocarbon exploration. It is critical, to determine the finer characterizations of the karst paleogeomorphology in order to identify favorable zones for exploration and development

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