Petrofacies and diagenesis of Thanetian Lockhart Limestone in the Upper Indus Basin (Pakistan): implications for the Ceno-Tethys Ocean

An integrated study based on petrography, SEM and XRD analysis has been carried out for petrofacies and diagenesis of the Lockhart Limestone in the Upper Indus Basin, Pakistan to decipher the paleo-environmental history during the mature development of the Ceno-Tethys Ocean. The Thanetian Lockhart Limestone has been divided into three petrofacies referred as: mudstone (CMF-I), mudstone to wackestone (CMF-II), and wackestone to packstone (CMF-III). These petrofacies deposited in the transgressive environment of the Ceno-Tethys Ocean along the toe of the carbonate platform's slope, slope, and shelf areas, respectively. In the Thanetian Limestone, three diagenetic stages indicated by their respective types of cementing material i.e., CMF-I demonstrates eogenesis with the appearance of drusy cement, CMF-II shows both mesogenesis and telogenesis, revealed by the blocky and rim cement, while CMF-III displays mesogenesis with the presence of blocky cement. These various grades of diagenesis are ultimate directly proportional to the ascending hierarchy of cement generations. Syngenetic-originated cubic pyrite and diagenetically formed framboidal pyrite are confined to the first microfacies (CMF- I), depicting that its deposition took place under a relatively deeper anoxic environment as compared to other microfacies. Compaction, cementation and micritization are the diagenetic impacts responsible for the destruction of primary porosity. While, in contrast, secondary porosity has been generated by epidiagenetic impacts like stylolites, fracturing, dolomitization and dissolution. Hence, these factors have enhanced the reservoir characteristics for Lockhart Limestone as a potential hydrocarbon reservoir rock.