Hydrothermal Dissolution of Deeply Buried Cambrian Dolomite Rocks and Porosity Generation: Integrated with Geological Studies and Reactive Transport Modeling in the Tarim Basin, China

Wenwen Wei,Daizhao Chen,Yixiong Qian,Hairuo Qing

doi:10.1155/2017/9562507

Abstract

The burial dissolution of carbonate rocks has long been an interesting topic of reservoir geologists. Integrated with geological studies and reactive transport modeling, this study investigated the Cambrian dolomites that were buried at depths up to 8408 m and still preserved a large amount of unfilled dissolution vugs from the borehole TS1 in the northern Tarim Basin. Studies indicate that these vugs were formed in association with fault-channeled hydrothermal fluids from greater depth through “retrograde dissolution” as the fluid temperature dropped during upward migration. The reactive transport modeling results suggest an important control of the vertical permeability of wall-rock on fluid and temperature patterns which, in turn, would control the spatial distribution of dissolving-originated porosity. The hydrothermal dissolution mainly occurred in dolomite wall-rocks with higher vertical permeability (extensive development of tensional fractures and connected pore spaces), producing additional dissolved porosity there during deep burial. This study implicates the importance of multidisciplinary approaches for understanding the burial/hydrothermal dissolution of dolomite rocks and predicting favourable deep/ultradeep carbonate reservoirs.

Highlights

It is commonly believed that about 50–60% of oil and gas reservoirs in the world occur in carbonate rocks [1]
We investigated the diagenetic processes and porosity development tied to fault-controlled hydrothermal fluids in the deep Cambrian dolomite reservoir revealed in the ultradeep borehole TS1 (∼8408 m deep) in Tarim Basin, based on the petrographic investigation, petrophysical data integrated with fluid reactive transport modeling using TOUGHREACT V2 [9]
The Cambrian dolomite strata penetrated by the deep well TS1 in the northern Tarim Basin are characterized by microbial reefs deposited on the platform margin

Summary

Introduction

It is commonly believed that about 50–60% of oil and gas reservoirs in the world occur in carbonate rocks [1]. Carbonate reservoirs are commonly heterogeneous as a result of complicated diagenetic processes which could have substantially modified the pore systems [2]. For this reason, understanding of spatial diagenetic differences and subsequent predicting of “sweet” reservoirs are quite a challenge for recovery of carbonate hydrocarbon resources. The borehole TS1, one of the deepest petroleum wells (to the depth of 8408 m) in the world, was drilled in the northern uplift of the Tarim Basin, northwestern China, and revealed a large number of dissolution pores and vugs, notably a downward increase in porosity, in the deeply buried Cambrian dolomites from 6884 to 8408 m deep [5]. Modeling the fluid property, pathway, and dissolution/precipitation is of great significance in understanding the formation of vuggy pore systems and controls in such a great depth, facilitating exploration for the ultradeep hydrocarbon reservoirs which were poorly constrained and understood in general

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