Diagenesis and Quality of Deep Cambrian Carbonate Reservoirs From the Tarim Basin, Northwestern China

Abstract

The Tarim Basin, largest petroleum basin in China, has more than 30 oil and gas fields producing from carbonate reservoirs, many under very large burial depths (6,000-8,000 m), high pressure and temperature. Quantitative petrographic analysis of two areas revealed the main factors controlling the quality of the deep carbonate reservoirs. Most of the original deposits were extensively dolomitized, but relicts and ‘ghosts’ preserved in many thin sections allow recognizing various primary constituents deposited under different conditions. Low-energy facies include siliciclastic mudrocks, calci-mudstones and wackestones. Moderate-to-high-energy facies are represented by peloidal, intraclastic and/or bioclastic packstones to grainstones. Few thin sections display organic films with crinkly lamination, suggestive of microbialites. The deposits were often extensively replaced by finely- to coarsely-crystalline dolomite, formed during several phases. Early dolomitization generated finely- to medium-crystalline dolomite, often zoned. A second phase formed dolomite overgrowths around zoned crystals and saddle dolomite. The latest phase precipitated radial ondulous, coarse dolomite crystals that overgrew and partially replaced saddle dolomite. The second and third dolomitization stages were precipitated under high temperature and pressure. The dolostones are locally replaced by microcrystalline quartz. The widespread occurrence of stylolites is related to the deep burial of the reservoirs. The rocks lack any primary porosity, but may display intense fracturing and dissolution, forming intercrystalline, fracture and vuggy secondary porosity. Vuggy and moldic porosity from the dissolution of bioclasts may be partially or totally filled by calcite, gypsum/anhydrite and/or coarse quartz. The diagenetic evolution and quality of the Cambrian reservoirs were strongly influenced by the ascent of hydrothermal fluids derived from the plutonic basement, and by the thermal maturation of underlying source rocks. However, the origin of the fluids responsible for early dolomite dissolution and silicification is not clear. Understanding the diagenetic evolution of the Cambrian reservoirs is essential for the construction of a genetic model for decreasing the risks of exploration for similar reservoirs in new areas of the Tarim Basin, and for increasing the recovery efficiency from producing fields.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018