Petrophysical and Mineralogical Study on Tight Formation of the Lower Indus Basin, Pakistan

Abstract

Significant amounts of hydrocarbons exist in carbonate reservoirs; however, these are difficult to develop due to several complexities. Pakistan possesses huge hydrocarbon potential within carbonate reservoirs mainly distributed in upper and lower Indus basins. The study was to characterize the carbonate rock intervals of the Jakkher group from Paleocene to Oligocene of lower Indus basin by collecting the comprehensive, high-quality data sets on petrophysical properties of carbonate rocks along with their mineralogy and microstructure. Experiments include but not limited to scanning electronic microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD), petrography, helium porosity, and steady-state gas permeability. Thin section, SEM-EDS, and XRD analyses revealed that the samples mostly contain carbon (C), calcium (Ca), and magnesium (Mg) as dominant elements along with aluminum (Al), silicon (Si), iron (Fe), and intermixes of clays and cementing materials. The analyses showed that: 1) Permeability and porosity data cross plotted were scattered. Similarly, the permeability versus slippage factor appears to be scattered, which shows the reflection of carbonate rock heterogeneity. 2) The permeability and clay mineralogy cross plots resulted in poor correlation. 3) Several diagenetic processes had influenced the quality of carbonates of the Jakkher group, such as pore dissolution, calcification, cementation, and compaction and grain fracturing. 4) Reservoir quality was mainly affected by inter-mixing of clay, cementation, presence of micrite muds, grain compactions, and overburden stresses. 5) The tight carbonate rocks were stress sensitive when permeability was measured under overburden stress. The analyses display that the Jakkher group carbonate reservoirs are tight and are considered to be of poor quality. Though thermal conductivity is one of the basic thermophysical properties of sedimentary rocks, data on the lower Indus basin are not available in public domain. Hence, measurements were made on several samples of Kirthar limestone, Laki limestone, Nari sandstone, and Nari siltstone collected from the lower Indus basin. The thermal conductivity of samples measured ranges from 0.76 to 3.46 W/m·K with a mean of 1.51 to 3.13 W/m·K. The results show that the thermal conductivity values of Nari siltstone, Laki limestone, and Kirthar limestone samples are relatively low with ranges of 1.58-2.24 W/m·K, 1.19-2.47 W/m·K, and 0.76-2.67 W/m·K, respectively. While the thermal conductivity of Nari sandstone samples exhibited relatively high values ranging from 2.98 to 3.46 W/m·K. It is apparent that the thermal properties considerably differ due to rock heterogeneity.