Unconventional Resource Potential Evaluation of Shale Facies of Sembar Formation, Lower Indus Basin, Pakistan

Abstract

The escalating global energy demand has heightened interest in unconventional hydrocarbon resources, particularly shale gas and shale oil. This study evaluates 100 well cuttings and core samples of the Sembar shale facies from five exploratory wells in the Lower Indus Basin of Pakistan using novel multi-analytical techniques. While previous studies relied on screening analyses, which were insufficient to fully assess the unconventional hydrocarbon potential of the Sembar Formation, this study integrates organic and inorganic geochemistry, petrology, microscopy, vibrational spectroscopy, and basin modeling to provide a comprehensive evaluation. The aim is to extensively characterize the Sembar shale's unconventional properties, including hydrocarbon potential, paleoenvironmental conditions, mineralogy, porosity, and burial history modeling. The screening results identified 23 samples with TOC values above 2 wt.%, suggesting good unconventional potential. A total of 17 samples of the Sembar shale from Jumman Shah-1, Palari Sarki-1, and Baki Ilkin-1 were identified as EHOM deposits with TOC values exceeding 3 wt.%. The Sembar shale primarily comprises type III kerogen, with occasional occurrences of type II, type II/III, and type IV kerogen. The Sembar shale exhibits varying maturity levels (mainly late oil window), with vitrinite reflectance values ranging from 0.71 to 1.64%. Basin modeling results demonstrate significant variation in transformation ratio (TR) across wells, ranging from 11 to 80%, reflecting differences in thermal maturity and geological settings. Biomarker results indicate the higher maturity of Sembar shale and a mixture of facies with varying contributions from higher plants, algae, plankton, and bacteria deposited in marine depositional settings that were predominantly suboxic and intermittently anoxic. Microfossils such as Radiolaria, Biserial Foram, Globotruncana, and Textularia further support the marine environment. The Sembar shale is classified into aluminumrich siliceous and mixed siliceous mudrocks. It exhibits characteristics of a passive continental rift tectonic setting, primarily sourced from intermediate igneous rocks. Favorable organic matter accumulation in the Sembar shale was facilitated by warm, humid climates, along with a combination of moderate terrigenous influx, higher paleobathymetry, and increased hydrothermal activity. Bulk mineral analysis identified quartz, kaolinite, and illite as the main minerals, with other minerals including montmorillonite, calcite, dolomite, siderite, feldspar, muscovite, biotite, sericite, zeolite, gypsum, pyrite, glauconite, magnetite, hematite, rutile, zircon, and tourmaline. The Sembar shale, with a brittleness index exceeding 0.48, is suitable for fracking and contains both matrix pores (e.g., interparticle, intraparticle, and organic-matter pores) and non-matrix pores, such as natural fractures, which enhance hydrocarbon storage mechanisms. Additionally, nanopores and nanochannels are present. CT scans revealed interconnected pores with porosity ranging from 0.97 to 5.09%, further highlighting its potential for hydrocarbon extraction. Core intervals from Baki Ilkin-1 met all cut-off parameters except hydrogen index (HI). It is concluded that four sweet spots (3388- 3390 m, 3390-3392 m, 3428-3430 m, and 3464-3466 m) in Palari Sarki-1 and two sweet spots (3170-3175 m and 3410-3415 m) in Jumman Shah-1 are optimal for Sembar shale oil and shale gas extraction, respectively. This study addresses key research gaps in accurately evaluating Pakistan's unconventional hydrocarbon resources through a multi-analytical approach, providing insights that aid in devising exploration strategies for similar geological conditions in Pakistan and elsewhere.