Discovery of a shoal-controlled karst dolomite reservoir in the Middle Permian Qixia Formation, northwestern Sichuan Basin, Southwest China

Abstract

The dolomites of the Middle Permian Qixia Formation have been important targets of natural gas exploration in the Sichuan Basin for decades. However, more and more exploration and research indicate that the formation of the reservoir might be related to karstification. To testify this hypothesis, we conduct comprehensive outcrop, core, and logging analyses based on a case study in the representative northwestern Sichuan Basin, which has obtained exploration breakthroughs recently. Results show that the Qixia dolomite reservoirs are mainly developed within fine-crystalline dolomites formed by a series of diagenetic modifications, which can be further divided into three types according to the macro-and micro-occurrences of dolomites: euhedralsubhedral crystalline dolomites in the quasi-stratiform karst system (mean porosity and permeability is 3.51% and 3.11 mD, respectively), euhedral-subhedral crystalline dolomites in the leopard porphyritic karst system (mean porosity and permeability is 3.36% and 1.22 mD, respectively), and allotriomorphic mosaic crystalline dolomites with residual parent rock fabrics (mean porosity and permeability is 0.94% and 0.92 mD, respectively). Their reservoir qualities decrease along the order. The formation mechanism of the reservoir is shoal-controlled karst. The preservation of residual intergranular pores within the thin-layer grainstones of shoal facies provides favorable channels for karst water. In the vadose zone, the heterogeneous dissolution within grainstones

Sedimentary responses to the Dongwu movement and the influence of the Emeishan mantle plume in Sichuan Basin, Southwest China: significance for petroleum geology

“…Wang et al (2013) and Tan et al (2015) proposed that a large-scale regression occurred during the late Qixia period, which caused extensive exposure of the upper Qixia Fm for a relatively long time, leading to well-developed karstification. Because this karstification occurred during the early diagenetic stage and its distribution is obviously controlled by the shoal facies, some researchers refer to it as eogenetic shoalcontrolled karstification (Tan et al 2015;Xiao et al 2018).…”

“…4c). Such patchy pore-cave systems, which are formed by dissolution, are usually filled by loose in situ or quasi-in situ carbonate sand (Tan et al 2015;Xiao et al 2018). Because of their good reservoir properties, these pore-cave systems are susceptible to dolomitization by later magnesium (Mg)rich fluids.…”

“…In recent years, it has been discovered that interlayer paleo-karstification and karst reservoirs also occur in the Qixia Fm (Wang et al 2013;Xiao et al 2018), which underlies the strata generally believed to be affected by the DM (i.e., the top of the Maokou Fm). In addition, the formation of dolostone (high-quality reservoir rocks) in the Qixia and Maokou Formations is considered to be closely related to tectonic-hydrothermal activity caused by the eruption of Emeishan Basalts and reactivated basement faulting between the Middle and Late Permian (Chen et al 2012;Jiang et al 2014;Liu et al 2016Liu et al , 2017.…”

Special Issue: Advances of petroleum exploration and geology research in West China's petroliferous basins

“…With increasing carbonate content, meso-and macropore volumes were observed to increase and decrease, respectively. Xiao et al (2018) describe a middle Permian shoal-controlled karstic dolomite reservoir in the northwestern Sichuan Basin. They divide fine-crystalline dolomites of the Qixia Formation into three groups: (1) euhedral-subhedral crystalline dolomite associated with a quasi-stratiform karst system (mean porosity ¼ 3.51%; mean permeability ¼ 3.11 mD); (2) euhedral-subhedral crystalline dolomite associated with a leopard porphyritic karst system (mean porosity ¼ 3.36%; mean permeability ¼ 1.22 mD); and (3) allotriomorphic mosaics of crystalline dolomite associated with relic parent rock fabrics (mean porosity ¼ 0.94%; mean permeability ¼ 0.92 mD).…”

An experimental study simulating the dissolution of gypsum rock

“…Previous research indicates that over 200 mg of gypsum rock is soluble in 100 g water at room temperatures. In contrast, the solubility of limestone and dolomite, which are commonly co-deposited with gypsum rock in saline sedimentary environments and have a great influence on the accumulation of oil and gas (Liu et al, 2017; Ma et al,2016; Wu et al, 2017; Xiao et al, 2017), is an order of magnitude less, at only 12 mg per 100 g of water. Therefore, the dissolution of gypsum rock is one of the key factors to consider when evaluating its distribution and preservation, and the wider implications of its sealing properties.…”