Organic geochemistry of crude oils from Oligocene reservoirs in the Salin sub-basin, Myanmar: Insights into source, maturity, and depositional environment

[Display omitted] •The Salin sub-basin is one of the largest petroliferous onshore sub-basins of Myanmar.•Different hydrocarbon seeps and oils occur along both eastern and western margins of the Central Basin.•Yenangyaung Oils (Group I) were originated from the lacustrine-marine source under the sub-oxic environment.•Chauk Oils (Group II) were generated from the fluvial/deltaic-marine source rocks under an oxic/sub-oxic condition.•Letpando Oils (Group III) were originated from the fluvial/deltaic-marine source rocks under an oxic environment. Fourteen crude oil samples were collected from the South (Yenangyaung: YNG), Central (Chauk: CHK), and North (Letpando: LPDO) oilfields in the Salin sub-basin, Myanmar. This study investigated the organic geochemical characterization of crude oils from early to late Oligocene reservoir rocks. The saturated and aromatic hydrocarbons of selected oil samples were analyzed by integration of gas chromatography and gas chromatography-mass spectrometry methods to evaluate their origin, depositional environment, and thermal maturity. All the oils have moderate to high API gravity, showing medium to light oils. Crude oils were classified into three groups based on their biomarker compositions. The group I oils (Okhmintaung Formation, upper Oligocene, YNG Oil Field) mainly originated from planktons/algal materials with less contribution of terrestrial input, deposited under sub-oxic environments (lacustrine-marine setting). The group II oils (Padaung Formation, middle Oligocene, CHK Oil Field) were significantly generated from planktons/algal sources with minor input from terrestrial materials, deposited in an oxic/sub-oxic environment (fluvial/deltaic-marine setting). Group III oils (Shwezettaw Formation, lower Oligocene, LPDO Oil Field) were derived from a mixed contribution of planktons/algal materials and terrestrial input accumulated under an oxic environment (fluvial/deltaic-marine setting). Group I and Group II oils have reached peak oil window, while Group III oils indicate the early oil window. The results of stable carbon isotopic compositions for all analyzed oil samples indicate that the source rocks were derived from marine algae, and terrestrial and/or freshwater algal materials. Oil-oil correlation suggested that Oil Groups exhibit mixed source characterization during hydrocarbon accumulation processes.