Multiphase dolomitization of deeply buried Cambrian petroleum reservoirs, Tarim Basin, north-west China

Cambrian dolostone reservoirs in the Tarim Basin, China, have significant potential for future discoveries of petroleum, although exploration and production planning is hampered by limited understanding of the occurrence and distribution of dolomite in such ancient rocks buried to nearly 8 km. The study herein accessed new drill core samples which provide an opportunity to understand the dolomitization process in deep basins and its impact on Cambrian carbonate reservoirs. This study documents the origin of the dolostone reservoirs using a combination of petrology, fluid-inclusion microthermometry, and stable and radiogenic-isotopes of outcrop and core samples. An initial microbial dolomitization event occurred in restricted lagoon environments and is characterized by depleted delta C-13 values. Dolomicrite from lagoonal and sabkha facies, some fabric-retentive dolomite and fabric-obliterative dolomite in the peloidal shoal and reef facies show the highest delta O-18 values. These dolomites represent relatively early reflux dolomitization. The local occurrence of K-feldspar in dolomicrite indicates that some radiogenic strontium was contributed via terrigenous input. Most fabric-retentive dolomite may have precipitated from seawater at slightly elevated temperatures, suggested by petrological and isotopic data. Most fabric-obliterative dolomite, and medium to coarse dolomite cement, formed between 90 degrees C and 130 degrees C from marine evaporitic brine. Saddle dolomite formed by hydrothermal dolomitization at temperatures up to 170 degrees C, and involved the mixing of connate brines with Sr-enriched hydrothermal fluids. Intercrystalline, moldic, and breccia porosities are due to the early stages of dolomitization. Macroscopic, intergranular, vuggy, fracture and dissolution porosity are due to burial-related dissolution and regional hydrothermal events. This work has shown that old (for example, Cambrian or even Precambrian) sucrosic dolomite with associated anhydrite, buried to as much as 8000 m, can still have a high potential for hosting substantial hydrocarbon resources and should be globally targeted for future exploration.