Assessment of hydrocarbon generation potential and thermal maturity of the offshore Mannar Basin, Sri Lanka

Amila Sandaruwan Ratnayake,Chaminda Wimal Kularathne,Yoshikazu Sampei

doi:10.1007/s13202-017-0408-1

Amila Sandaruwan Ratnayake, Chaminda Wimal Kularathne + Show 1 more

Open Access

https://doi.org/10.1007/s13202-017-0408-1

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Abstract

The Mannar Basin is an under-explored offshore sedimentary basin in terms of petroleum geology. The Cretaceous to Paleogene cutting samples were collected in three offshore exploration wells (i.e. the Dorado North, Dorado and Barracuda) in the Mannar Basin. In this study, kerogen type, quantity and thermal maturity of sedimentary organic matter were ascertained using Rock–Eval pyrolysis and vitrinite reflectance analyses. In addition, 1-D basin modelling was used to analyse the timing of hydrocarbon generation in the Mannar Basin. Total organic carbon (TOC) contents and total hydrocarbon potential are higher in the Cretaceous calcareous mudstones than in the Paleogene calcareous silty/sandy mudstones. TOC contents show a negative correlation with oxygen index. Hydrogen index and oxygen index values range from 77 to 785 mg HC/g TOC and from 25 to 165 mg HC/g TOC, respectively. Organic matters were identified as mixed Type II–III and Type III kerogen. Maturity expressed in term of Tmax and vitrinite reflectance range from 332 to 456 °C and from 0.26 to 1.49%, respectively. It indicates both immature and mature sediments. In general, maturity values are relatively high in the Late Cretaceous sediments (Tmax, from 332 to 456 °C, average = 423 °C ± 25) compared to the Paleogene sediments (Tmax, from 388 to 431 °C, average 419 °C ± 10). Geochemical proxies show that oil and gas prone (Type II–III) and gas prone (Type III) kerogen-rich Late Cretaceous sediments of the Dorado and Barracuda wells have principally achieved maturity levels for oil window. Therefore, it suggests incomplete thermal conversion for the wet gas generation in the northeast part of the Mannar Basin. In the 1-D basin modelling, the maximum hydrocarbon generation was observed during the Neogene Period in the Late Cretaceous sediments of the deeper Barracuda well.

Highlights

The Mannar Basin is one of the largest geological provinces for the hydrocarbon exploration in Sri Lanka (Fig. 1)
Total hydrocarbon potential is higher in the Cretaceous sediments than in the Paleogene sediments
A significant amount of the analysed samples recorded more than 1.0 wt% of Total organic carbon (TOC), and more than 4 mg HC/g rock of S2 yield in Rock–Eval pyrolysis

Summary

Introduction

The Mannar Basin is one of the largest geological provinces for the hydrocarbon exploration in Sri Lanka (Fig. 1). Rock–Eval pyrolysis is the most widely applied standard method in petroleum industry to characterise hydrocarbon generation potential (quantity), types of kerogen, types of primary hydrocarbons (oil and gas) and thermal maturity of sedimentary organic matter Journal of Petroleum Exploration and Production Technology (2018) 8:641–654 an optical thermal maturity indicator of sedimentary organic matter since Silurian (Ritter 1984; Sweeney and Burnham 1990; Uysal et al 2000; Mani et al 2015). The current study is based on the detailed interpretation of qualitative and quantitative aspects of hydrocarbon generation potential and thermal transformation of organic matter using Rock–Eval pyrolysis and vitrinite reflectance (Ro%) data. Compositional kinetics models were considered to determine the evolution of thermal maturity and timing of hydrocarbon generation. This study provides basic information for the hydrocarbon exploration works in Sri Lanka

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