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

Abstract

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 δ13C 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 δ18O 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°C and 130°C from marine evaporitic brine. Saddle dolomite formed by hydrothermal dolomitization at temperatures up to 170°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.

Petroleum reservoir quality prediction: overview and contrasting approaches from sandstone and carbonate communities

“…Although many conceptual models of dolomitization have been presented, it is now widely concluded that platform-scale dolomitization is most likely to occur from the reflux of evaporated seawater, since this is the most volumetrically abundant source of fluids with a high-Mg/Ca ratio (Adams & Rhodes 1960;Jiang et al 2016). Reflux is interpreted to occur owing to the formation of brine pools on the platform top, particularly within arid environments.…”

Multiphase Calcite Cementation and Fluids Evolution of a Deeply Buried Carbonate Reservoir in the Upper Ordovician Lianglitag Formation, Tahe Oilfield, Tarim Basin, NW China

“…The structural history of the Western Canada basin is broadly analogous to that of the Tahe area. Thus, it is possible for a similar model of water flow to have occurred in the study area as previously proposed by Li et al [15], Jiang et al [61], and Jiang et al [62].…”

“…Unfortunately, the data are not enough. Combined with previous works in Tarim Basin [15,61,62], C6 was probably closely related to deeply recycled meteoric water. The reduced water coexisted with highly mature oils with blue-white UV fluorescence emission colors.…”

Reconstructing large‐scale karst paleogeomorphology at the top of the Ordovician in the Ordos Basin, China: Control on natural gas accumulation and paleogeographic implications

“…Entry of Fe 2+ from fluid into the crystal lattice occurs mainly under reducing conditions . Furthermore, negative O isotope anomalies in the ankerite reflect the effect of high temperature . Therefore, it is inferred that the ankerites represent the products of fillings during the burial stage and that they are indicators of good porosity preservation before reservoir space was partially filled in the northern karst slope.…”