Geochemical characteristics and diagenetic fluids of dolomite reservoirs in the Huanglong Formation, Eastern Sichuan Basin, China

Abstract

Based on a comprehensive study of texture, diagenetic behavior and evolution of dolomite in the Huanglong Formation, trace (e.g., Fe, Mn and Sr) and rare earth element (REE) geochemistry, and isotopic characteristics (e.g., C, O and Sr), four types of diagenetic fluids are identified in the Huanglong Formation dolomite reservoirs of the Eastern Sichuan Basin, China: 1): marine-derived pore waters in the marine diagenetic environment, 2) sabkha compaction brine conserved in the early shallow-buried diagenetic environment, 3) strongly-oxidizing low-temperature meteoric water in the seepage-subsurface flow diagenetic environment, and 4) strongly reducing deeply seated mixed hot brine in the middle and deep burial diagenetic environment. The fluids developed hereditarily from one environment to another, which resulted in its respective characteristics. Fluid characteristics play an important role in the development of dolomite reservoirs: 1) dolomitization by marine-derived pore water in the quasi-syngenetic stage did not form an effective reservoir; 2) early diagenetic burial dolomitization by the sabkha compaction brine formed the basis for reservoir development; 3) meteoric water karstification in the paleo-epidiagenetic stage expanded both the distribution and the size of the reservoirs, and improved the reservoir quality; 4) deep-burial dissolution and tectonic fracturing in the reburial diagenetic stage further improved reservoir porosity and permeability.