Facies Architecture, Depositional Environment and Hydrocarbon Potential of Paleocene Succession of the Eastern Sulaiman Fold & Thrust Belt, Pakistan

Abstract

The Paleocene succession, consisting of Ranikot Formation and Dungan Formation, is outcropped very well in the Sulaiman Fold & Thrust Belt of Pakistan. The study area is located in Musakhel District and Loralai district of Balochistan, in the eastern to central Sulaiman Fold Belt of Pakistan. The aim of the present thesis is to study the facies architecture, depositional environment and hydrocarbon potential of the Paleocene succession in the proposed area of the Sulaiman Fold & Thrust Belt, Pakistan, which was deposited during the early phase of India-Eurasia collision. The main objectives of the project included the vertical and lateral distribution of facies and depositional environment of Ranikot Formation and Dungan Formation at selected sections in the study area. Beside it, the petrographic study determined the source rock and geochemical analysis recognized petroleum source rock potential of the Paleocene sequence. Extensive field work was carried out to collect necessary data that included detailed description of various rock units where two (02) stratigraphic sections were measured. These stratigraphic sections are 30 kilometers away from each other, namely Kingri Section and Sra Dhaka Section. Sedimentray logs and field observations enabled us to interpret the facies architecture and depositional model of the succession. Length of Sra Dhaka Section is 1100 meters (Ranikot Formation 800 m and Dungan Formation 300 m) and Kingri section is 660 meters (Ranikot Formation is 190 m and Dungan Formation is 470 m thick). These two sections are outcropped and measured in two different anticline folds. The rocks in Sra Dhaka anticline are exposed/outcropped till Parh Limestone and in Kingri anticline these rocks are exposed till Ranikot Formation. Owing to less exposure of Kingri anticline the Ranikot Formation is only 190m thick. In the Sra Dhaka Section, the Ranikot Formation is characterized mainly by mudstones interbedded with thin sandstones and rare limestones, described into three facies that include: (1) Slumped mudstone interbedded sandstone facies (2) Channelized sandstone with interbedded mudstone facies and (3) Thick mudstone with interbedded thin sandstone facies. The Dungan Formation is characterized by mudstones interbedded with very few limestone beds near base and on the contact with underlying Ranikot Formation and can be put into a single facies; (1) Mudstone with few interbedded limestone facies. Within the Kingri Section, the Ranikot Formation is characterized by mudstones interbedded with sandstones having seven facies that include: (1) Mudstone with interbedded sandstone facies (2) Sandstone with interbedded mudstone facies (3) Thick and erosive sandstone facies (4) Heterolithic mudstone-sandstone facies (5) Channelized sandstone with interbedded mudstone facies (6) Mudstone with interbeds of tabular sandstone facies, and (7) Mudstone with few interbedded glauconitic sandstone facies. The Dungan Formation in this Section is characterised by mudstones interbedded with subordinate limestone beds that are present prominently in the central part of the formation. There are few limestone beds in the formation's upper and lower parts but the overall formation is mudstone dominated. It is divided into four facies that include: (1) Calcareous mudstone with few interbedded limestone facies, (2) Limestone with interbedded mudstone facies (3) Mudstone with few interbedded limestone facies and (4) Calcareous shale with few interbedded sandstone facies. The interpreted depositional environment of the Paleocene succession (mostly Ranikot Formation) in the study area shows slope to shelf and shallow marine environments with an overall shoaling up trend. Carbonates of the Dungan Formation of Kingri Section indicate shallow marine conditions. Twenty five (25) samples from Kingri Section and sixteen (16) samples from Sra Dhaka Section were collected for petrographic, geochemical and biostratigraphic studies. Petrographic investigation was carried out on twenty (20) samples (13 of sandstones and 7 of limestones) from both geological sections. Thin sections were prepared and studied under the polarizing microscope in the petroghrapic lab of Bahria Universty, Islamabad campus. Acquired results for sandstone classification were plotted on triangular diagram where the samples from Ranikot Formation and Dungan Formation of the Kingri Section fall into the field of sub-lithic arenite and two sandstone samples of Sra Dhaka Section plot into the field of sub-arkose and two samples into the quartz arenite field. For point counting purpose six (06) representative samples (medium to coarse grained) of the sandstone, one (01) from Ranikot Formation, one (01) from Dungan Formation of Kingri Section and four (04) samples from Ranikot Formation of Sra Dhaka Section were selected for modal analysis. Five hundred (500) points per sample were counted and the counting interval was 1/3 mm and, traverses were kept 1 mm apart to prevent multiple counts of the very coarse components. Sandstone petrography and debris modes, displayed in discrimination diagrams, suggest recycled orogen as source area of Kingri Section and Craton Interior as source area of Sra Dhaka Section. The palacoflow direction is toward west and northwest, suggesting the Indian basement as source area. Seven 07 representative samples of the limestone studied and classified according to Dunham's classification. Out of these (07) samples, one (1) sample is from Ranikot Formation (bioclastic grainstone) and five samples (5) from Dungan Formation. (Foraminiferal packstones, planktonic wackstone, bioclastic wackstone-packstone and foraminiferal mudstone-wackstone) of Kingri Section and one (1) sample from Dungan Formation (planktonic mudstone-wackstone) of Sra Dhaka Section, Eight (8) samples of mudstone and shale were studied for nano-fossils at University of Peshawar but only a single sample yielded Marzovilla of Paleocene age. In order to carry out geochemical studies, 24 selected samples (9 from Ranikot Formation and 15 from Dungan Formation) were analysed for Total Organic Carbon (TOC), using TOC Analyzer (Eltra 580A) and ten (10) samples (6 from Ranikot Formation and 4 from Dungan Formation) were run on Pyrolysis instrument Rock Eval- 6 to determine Free Hydrocarbons, Source Potential, Hydrogen Index, Oxygen Index and Maturity. The TOC values of Ranikot Formation ranges from 0.10% to 0.77% and that of Dungan Formation from 0.10% to 0.36%, categorizing poor to fair. The Ranikot Formation TOC is comparatively better than Dungan Formation. SI values of Rock Eval range from 0.01-0.06mg/g rock and S2 from 0.00 to 0.82 mg/g rock, showing that all the samples fall in poor category. Samples from both sections represent Type IV kerogen which has no or less potential to generate oil and gas. Poor TOC values are also responsible for the weak genetic potential of the samples. Similarly, production index values show gas and some expected oil products. Both the sections indicate poor potential of hydrocarbon generation and the majority of the samples lie in the mature and post- mature zone.