Petrological and geochemical constraints on diagenesis and deep burial dissolution of the Ordovician carbonate reservoirs in the Tazhong area, Tarim Basin, NW China

作者：Jia, Lianqi; Cai, Chunfang^*; Jiang, Lei; Zhang, Ke; Li, Hongxia; Zhang, Wen

来源：Marine and Petroleum Geology, 2016, 78: 271-290.

DOI：10.1016/j.marpetgeo.2016.09.031

• Dolomite
• Fluid inclusion
• Thermochemical sulfate reduction (TSR)
• Porosity
• Deeply burial diagenesis
• Tazhong area

Deeply buried (4500-7000 m) Ordovician carbonate reservoirs in the Tazhong area, Tarim Basin, NW China show obvious heterogeneity with porosity from null in limestones and sweet dolostones to 27.8% in sour dolostones, from which economically important oils, sour gas and condensates are currently being produced. Petrographic features, C,O, Sr isotopes were determined, and fluid inclusions were analyzed on diagenetic calcite, dolomite and barite from Ordovician reservoirs to understand controls on the porosity distribution. Ordovician carbonate reservoirs in the Tazhong area are controlled mainly by initial sedimentary environments and eo-genetic and near-surface diagenetic processes. However, vugs and pores generated from eogenetic and telogenetic meteoric dissolution were observed to have partially been destroyed due to subsequent compaction, filling and cementation. In some locations or wells (especially ZG5-ZG7 Oilfield nearby ZG5 Fault), burial diagenesis (e.g. thermochemical sulfate reduction, TSR) probably played an important role in quality improvement towards high-quality reservoirs. C2 calcite and dolomite cements and barite have fluid inclusions homogenization temperatures (Ths) from 86 to 113 degrees C, from 96 to 128 C and from 128 to 151 degrees C, respectively. We observed petrographically corroded edges of these high-temperature minerals with oil inclusions, indicating the dissolution must have occurred under deep-burial conditions. The occurrence of TSR within Ordovician carbonate reservoirs is supported by C3 calcite replacement of barite, and the association of sulfur species including pyrite, anhydrite or barite and elemental sulfur with hydrocarbon and C-12-rich (as low as 7.2 parts per thousand V-PDB) C3 calcite with elevated Ths (135-153 degrees C). The TSR may have induced burial dissolution of dolomite and thus probably improved porosity of the sour dolostones reservoirs at least in some locations. In contrast, no significant burial dissolution occurred in limestone reservoirs and non-TSR dolostone reservoirs. The deeply buried sour dolostone reservoirs may therefore be potential exploration targets in Tarim Basin or elsewhere in the world.

• 出版日期2016-12
• 单位中国石油天然气股份有限公司勘探开发研究院; 中国科学院地质与地球物理研究所; 中国科学院大学; 长江大学; 中国科学院地质与地球物理研究所兰州油气资源研究中心