Diagenetic Setting, Dolomitization and Reservoir Characterization of Late Cretaceous Kawagarh Formation, Khanpur Dam section, Hazara, Pakistan

Abstract

The Kawagarh Formation is well exposed in Hazara basin in different sections. Due to deep depositional settings, the Kawagarh Formation is least appealing to geologists to investigate it as a hydrocarbon reservoir. In the present study, the diagenetic settings of Kawagarh Formation were chronologically studied to interpret its diagenetic history and the effect of different diagenetic phases on the reservoir potential. The dolomitization is also studied in depth to use it as a key for its reservoir potential. Kawagarh Formation is sampled at Khanpur Dam Section for porosity analysis. The samples were taken from limestone and dolomite facies randomly. The dolomites are in the form of veins and well developed thick size beds. These dolomites are secondary in nature which is hosted by fractures and joints of limestone, which affect about 25% of limestone facies. At outcrop scale different types of dolomites are recognised on the basis of color and texture, yellowish fine-grained, brown blackish coarse-grained in the top portion and saddle dolomites. In petrographic study partial and complete dolomitization are observed. On the basis of crystal sizes and geometry different types of dolomites are recognized which are; (1) fine crystalline planar-euhedral dolomite, (2) medium crystalline planar-subhedral dolomite, (3) medium crystalline non-planar-anhedral dolomite, (4) coarse crystalline planar-subhedral dolomite, (5) coarse crystalline, non-planar-anhedral dolomite and (6) saddle dolomites (SD1). In petrographic study, high inclusions and disturbance are observed at the surfaces of dolomitic rhombs which indicate low Mg replacement or dedolomitization phenomena. On image J porosity analysis, the porosity is found of limestone and dolomitic samples. In limestone facies which is mostly, non-laminated mudstone has very low up to 2 to 3% in the form of vugs and fractures. In dolomitic facies, the porosity is ranging from 5% to 14%. In most of the dolomitic samples, the porosity is round about 5%. From the structural analysis and the study of other carbonate formations, it is concluded that this dolomitization occurs in the Kawagarh Formation along the Khui da Maira fault. Near the fault, the dolomites occur in the irregular pattern, but as we move away from faulted section, the dolomites are converted into horizontal veins and beds. The dolomitization model has been evaluated for better understanding of the phenomenon of dolomitization.