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Paleokarst System Controlled by Previous HitUnconformitiesNext Hit, Paleogeomorphology and Faults: A Case Study From the Tazhong Area, Tarim Basin, China

Abstract

The plaleokarst reservoir in the Lower-Middle Ordovician Yingshan Formation is one of the most important fields for oil and gas exploration in the Tazhong area, Tarim Basin. This study focuses on the paleokarst structure and its various control factors, and conducts an in-depth assessment based on a variety of information about cores, thin sections, well logging, seismic profile, etc., and also exploring the advantages of borehole image logs. The paleokarst structure of the Yingshan Formation consists of six imaging facies and fifteen micro-facies. Based on the analysis of the composite imaging facies of the single well paleokarst structure, it is found that two sets of paleokarst systems have developed in the Yingshan Formation, one is located in the first member of the Yingshan Formation, and the other is located in the second member of the Yingshan Formation. In the Tazhong area, the factors that control paleokarst development include Previous HitunconformitiesNext Hit, paleogeomorphology, and faults. (I)The Yingshan Formation suffered extensive denudation in the Late Ordovician, which created the Previous HitunconformitiesNext Hit on the top of the formation. Statistics show that nearly half of the caves are distributed within 30m beneath the Previous HitunconformitiesTop, and the number of caves decreases as the distance to the surface increased. (II) The analysis of paleogeomorphology shows that when looking in the direction of Tazhong No.1 fault, the denudation is strongest in the Southern Karst highland area, fluctuations in the terrain are relatively severe and the vertical zonality of the Karst is significant. Because it is a lateral groundwater recharge area, the Middle Karst slope area is the area with the most developed underflow caves and fractures with higher karst intensity. (III) Two sets of faults developed in the Tazhong area. A series of high angle strike-slip faults that formed in the Northeast during the Hercynian mostly intersected with Tazhong No.1 fault. The representative beaded seismic reflections of the paleokarst system developed near those faults, especially in the areas between two strike-slip faults. Faults in the Early-Middle Caledonian not only triggered the movement of the paleogeomorphology, but also improved the permeability of the formation to a great extent. These research results provide an important basis for subsequent research on establishing a multiple paleokarst model.