Hydrothermal silicification and its impact on Lower–Middle Ordovician carbonates in Shunnan area, Tarim Basin, NW China

Abstract

The Shunnan area is a newly discovered hydrocarbon field in Shuntuoguole lower uplift, in which the host rocks are Lower–Middle Ordovician carbonates. The carbonate reservoirs are well affected by hydrothermal silicification with well‐preserved intercrystalline pore of replacement quartz and megaquartz cements. Based on the petrography, pore types, and diagenetic processes, this research evaluated the reservoir quality and re‐constructed the porosity evolution in different diagenetic stages. Samples are collected from 7‐cored wells distributed along NNE‐ or ENE‐trending sinistral strike‐slip faults. Nine types of lithofacies were identified in the study area, including mudstone, bioclast wackestone, peloidal bioclast packstone, boundstone, peloidal grainstone, ooid grainstone, slightly silicified limestone (silica less than 25%), strongly silicified limestone (silica around 50%), and chert. Chert has better porosity (average 21.3%) and permeability (average 44.7 mD) than other lithofacies. The main reservoir pores are mainly developed in forms of fractures, intercrystalline replacement quartz and dissolved vugs. The porosity evolution shows that the primary porosity was limited due to cementation and compaction, and the earlier secondary porosity was occlusive as a result of coarse calcite cementation. Therefore, the precursor limestones may be tight, and the reservoir quality of the Lower–Middle Ordovician carbonates was controlled by hydrothermal silicification. During the hydrothermal silicification event, the burial depth of the Lower–Middle Ordovician carbonates was more than 3,000 m and their stratum temperature was higher than 120°C, representing a deep burial condition. The statistical data of porosity‐depth/temperature trends for carbonates suggest that the early secondary pores would be significantly reduced when the burial temperature reaches 100°C (or depth of strata reaches 3,000 m). Therefore, the dissolved vugs in strongly silicified limestone or hydrothermal chert are caused by hydrothermal silicification and not inherited from precursor carbonate rocks.