Characteristics and formation mechanisms of silicified carbonate reservoirs in well SN4 of the Tarim Basin

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 87 Sr/ 86 Sr 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 d 13C 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

Intense silicification of Ordovician carbonates in the Tarim Basin: Constraints from fluid inclusion Rb–Sr isotope dating and geochemistry of quartz

“…The indiscriminate microthermometries for matrix and cement quartz (Figure a; Table ) indicate that the matrix quartz crystals were probably formed from the similar compositional fluids related to the Yanshanian orogeny. This inference was also corroborated by the large overlap of δ 18 O isotopic compositions of matrix and cement quartz (You et al., ).…”

“…An unconventional hydrocarbon‐producing silicified carbonate reservoir (quartz > 90%) was unveiled in the Middle Ordovician fractured limestones within a strike‐slip fault zone in the Well SN4, Tazhong Uplift, Tarim Basin (Figure ). Several interpretations have been proposed to explain the occurrence of silicified carbonates (Chen, Lu, Cao, Han, & Yun, ; Lu et al., ; Qi, ; You et al., ). However, the geological events responsible for the massive carbonate silicification are poorly constrained and have remained controversial without reliable geochronological data.…”

Types of Fluid Alteration and Developing Mechanism of Deep Marine Carbonate Reservoirs

“…Particularly in the SN4 well, strong silicification related to hydrothermal in the Ordovician Yingshan Formation limestone reservoir (6668-6681 m) was revealed. The porosity of the silicified reservoir is 3-20.5%, much higher than that of the limestone reservoir (porosity of 1.4-1.6%) without hydrothermal alteration [62]. A high-yielding gas flow of 45 8 × 10 4 m 3 /day was produced from the silicified reservoir.…”

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

“…Fluids within the reservoir display an atypical distribution, with oil occurring above gas and water, which were locally produced in gas reservoirs in relation to the dynamic accumulation of oil and gas. You et al (2018) present a case study of well SN4 in the Tarim Basin and investigate the characteristics and formation mechanisms of silicified carbonate reservoirs. They divide the reservoirs into a lower section of silicified carbonate, a middle section of limestone, and an upper section of silicified carbonate.…”