Abstract
High-yield natural gas was discovered in well SN4 in the Ordovician Yingshan Formation in the Tarim Basin. The gas is found in unusual, silicified, carbonate reservoirs. According to the degree of silicification, the silicified reservoirs can be divided into a lower section of silicified carbonates, a middle section of limestone, and an upper section of silicified carbonates. The silicified carbonates are mainly composed of quartz and calcite, in which the reservoir space mostly occurs as vugs, inter-crystalline pores of quartz, and partial fractures. Porosity varies widely, ranging from 3 to 20.5% with strong heterogeneity. The homogenization temperatures of fluid inclusions in quartz and calcite show that the silicification temperatures were 150–190°C, with characteristics of high temperature/low salinity and low temperature/high salinity. The 87Sr/86Sr ratios of secondary calcite are 0.709336–0.709732, which are significantly higher than that of concurrent seawater, indicating that the hydrothermal fluid originated from the deep clastic strata or the basement (sialic rock). The δ13C values of the secondary calcite are similar to that of the surrounding limestone, indicating that the carbon in the secondary calcite is derived from the limestone strata, and that the secondary calcite is the product of dissolution and re-precipitation resulting from interaction between the silica-bearing hydrothermal fluids and surrounding limestones. The silicification of silica-bearing hydrothermal fluid was significantly controlled by strike-slip faults. The fluids ascending along the fault zone and branch faults interacted with the surrounding limestone in the Yingshan Formation. As a result, a large amount of quartz and secondary calcite were formed together with various types of secondary pores, resulting in excellent reservoirs.
Highlights
There are few studies considering silicified carbonates as reservoirs in hydrocarbon-rich sedimentary basins. Packard et al (2001) reported a hydrothermal, chert reservoir in the Parkland Field in the Western Canada Basin, Canada (Packard et al, 2001)
The silica-bearing fluid replaces limestone on both sides of the fractures and forms replacement quartz in the matrix, which occurs as euhedral-subhedral quartz (R-Qtz)
The space in fractures and vugs are filled with columnar euhedral quartz (FQtz) and secondary calcite (E-Cal and gigantic calcite (G-Cal)), and the surrounding rocks are altered to silicified carbonates, which mainly consists of quartz and calcite, where the quartz content can be up to 90% or more
Summary
There are few studies considering silicified carbonates as reservoirs in hydrocarbon-rich sedimentary basins. Packard et al (2001) reported a hydrothermal, chert reservoir in the Parkland Field in the Western Canada Basin, Canada (Packard et al, 2001). There are few studies considering silicified carbonates as reservoirs in hydrocarbon-rich sedimentary basins. The pre-salt carbonates in the Kwanza Basin of Angola underwent hydrothermal silicification, such that the silicified carbonate reservoir showed porosity of up to 15% and permeability up to 100 mD (Poros et al, 2017). In 2013, silicified carbonate reservoirs were discovered in the Yingshan Formation of well SN4 in the Tarim Basin (6668.81–6681 m), and a high-yield gas flow of approximately 40 Â 104 m3 per day was tested (Wang et al, 2014a). Understanding the development characteristics, formation mechanisms, and distribution of such silicified reservoirs is critical to oil and gas exploration in the northern area of the Tazhong No 1 Fault in the Tarim Basin
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