Reservoir Characterization of Lacustrine Argillaceous Dolomite with Trona Interbeds

Yao, Guangqing¹, Fengde Zhou², Timothy R. Carr³, Shangru Sun⁴ (1) China University of Geosciences, Wuhan,Hubei, China (2) China University of Geosciences, Wuhan, China (3) Kansas Geological Survey, University of Kansas, Lawrence, KS (4) Henan oilfield company , Sinopec, Nanyang, China

In the center of Tertiary lake-basin within the Biyang depression of the Henan Province, central China, a suite of thick dolomitic units totaling more than 1500 meters was deposited by a perennial alkalescence saline depositional system (Hetaoyuan Formation). The lithic succession of shale (mudstone)-dolomitic marlstone - argillaceous dolomite- dolomite –trona records the procession from freshwater to increasingly salty water. In subsurface high quality trona ore was developed for commercial purposes. Reservoir whole-core data indicate fracturing and dissolution is present in the reservoirs. However, until this study, the dolomite reservoir quality and the potential were unclear. This study uses integrative techniques to characterize the dolomite reservoir. Available techniques and methods include analysis of sedimentary facies, high-resolution sequence stratigraphy, diagenesis and stress field. Fifteen short-term cycles are divided in sections and provide the basis to construct a rhythm-cycle record model of the dolomitic series. Detailed sections and sub-surface facies maps provide reliable indices of reservoir properties and the distribution of trona layers. Associated with the fractures, dolomite dissolution is identified through core and log analysis. The size of secondary pore space ranges from millimeter to centimeter scale. Based on well-bore deformation, rock acoustic emission testing, structural surface curvature analysis and simulation of the distribution and density of potential fracture system, the ancient maximum stress direction is northeast-southwest. Results indicate the dolomite units of the Hetaoyuan Formation have the potential for oil and gas accumulation with good reservoir quality resulting from a dual system of fracture and dissolution porosity.