DOLOMITE HOSTED IN THE MIDDLE TRIASSIC CARBONATE PLATFORM OF THE NANPANJIANG BASIN, SOUTH CHINA: EFFECTS OF DOLOMITIZATION PROCESSES ON PORE TYPES

Understanding the dolomitization process is crucial for predicting the distribution, geometry and textural heterogeneity of dolomite reservoirs. We present a case study of massive dolomite (MD) hosted in the Triassic Yangtze carbonate platform in the Nanpanjiang Basin of south China in order to understand its dolomitization processes, to compare against existing conceptual models, and to provide better predictive models for variations in heterogeneous carbonate rocks between wells.

The Yangtze platform at Guanling exposes MD in a continuous 2D platform-to-basin transect in the Nanpanjiang Basin. MD is present in the platform interior, slope and basin margin except for the marginal reef. MD in the platform interior overprints and cuts across peritidal cyclic facies. The MD geobody thins downward into older rock units of the platform interior. 3 stratiform dolomitized units subparallel to bedding occur in the slope. They generally taper out basinward, but several tapered dolomitization fronts terminate toward the platform. Dolomitization is pervasive in the platform interior, but in the slope it preferentially replaces debris flow units over pelagic lime mudstone.

3 types of dolomite texture are recognized: 1) dolomicrite (DM) forming peritidal cyclic cap; 2) fabric-retentive/-obliterative, fine to medium crystalline dolomite (FMCD); 3) dolomite cements (DC). The main porosity associated with the dolomite occurs in vugs where sub- to euheral dolomite cements line the pore wall. Subordinate interparticle porosity is more common in the platform interior where packstone is dolomitized. 3 episodes of dolomitization are interpreted. First, peritidal DM was initiated by reflux of normal-temperature sea water (~25-30^oC). The 2nd phase is represented by FMCD containing single-phase fluid inclusions (<55 ^oC) and low concentration of Fe, Mn and Sr, which might be related with reflux of diagenetic fluids of slightly higher salinity. DC associated with burial brine record the last episode of dolomitization, which is characterized by internal high-salinity of two-phase inclusions that homogenize at 63 to 142 ^oC. Ongoing measurements on C, O and Sr isotopes will provide key elements for deciphering the origin of the dolomitizing fluids and for further constraining the dolomitization processes.