Thermodynamic analyses of dolomite dissolution and prediction of the zones of secondary porosity: A case study of the tight tuffaceous dolomite reservoir of the second member, Permian Lucaogou Formation, Santanghu Basin, NW China

Abstract

The vertical distribution and geological origin of the zones of secondary porosity have been studied in the tight tuffaceous dolomites of the second member of Permian Lucaogou Formation, Santanghu Basin, Xinjiang, China. The lateral distribution of secondary porosity zones is predicted using the thermodynamic method. Three secondary porosity zones were identified in Malang-Tiaohu Sag. The secondary porosities were formed through reservoir dissolution by acids, which include organic acids generated from decarboxylation of kerogen and inorganic acids generated from the clay mineral transformations. Gibbs free energy increments of dissolution reactions for different minerals are calculated under various pressures and temperatures to investigate the lateral distribution of secondary porosity zones, with respect to the litho-facies distribution of the second member of the Lucaogou Formation. Calculation result shows that 1) deeply buried dolomite strata are most prone to be dissolved and 2) secondary pores were mainly formed in tuffaceous dolomites. In general, the most developed zones of secondary porosity with favorable tight oil reservoir potentials are located in the central Malang-Tiaohu Sag and over the high-quality source rocks that are semi-deep to deep lacustrine facies in origin. Key words: Santanghu Basin, tight oil, dolomite dissolution, free energy increment, secondary porosity zone