ORGANIC GEOCHEMISTRY OF OIL AND NATURAL GAS IN THE WEST DIKIRNIS AND EL-TAMAD FIELDS, ONSHORE NILE DELTA, EGYPT: INTERPRETATION OF POTENTIAL SOURCE ROCKS

M. Leila^a,b* and A. Moscariello^a

^a Department of Earth Sciences, University of Geneva, Switzerland.

^b Department of Geology, Mansoura University, Mansoura, Egypt.

^* Author for correspondence, mahmoud.leila@etu.unige.ch

Key words: Nile Delta, Egypt, source rocks, oil, West Dikirnis field, El-Tamad field, wet gas, biomarkers, Messinian, Qawasim Formation.

Crude oil in the West Dikirnis field in the northern onshore Nile Delta, Egypt, occurs in the poorly-sorted Miocene sandstones of the Qawasim Formation. The geochemical composition and source of this oil is investigated in this paper. The reservoir sandstones are overlain by mudstones in the upper part of the Qawasim Formation and in the overlying Pliocene Kafr El-Sheikh Formation. However TOC and Rock-Eval analyses of these mudstones indicate that they have little potential to generate hydrocarbons, and mudstone extracts show little similarity in terms of biomarker compositions to the reservoired oils. The oils at West Dikirnis are interpreted to have been derived from an Upper Cretaceous -- Lower Tertiary terrigenous, clay-rich source rock, and to have migrated up along steeply-dipping faults to the Qawasim sandstones reservoir. This interpretation is supported by the high C₂₉/C₂₇ sterane, diasterane/sterane, hopane/sterane and oleanane/C₃₀ hopane ratios in the oils. Biomarker-based maturity indicators (Ts/Tm, moretanes/hopanes and C₃₂ homohopanes S/S+R) suggest that oil expulsion occurred before the source rock reached peak maturity. Previous studies have shown that the Upper Cretaceous – Lower Tertiary source rock is widely distributed throughout the on- and offshore Nile Delta.

A wet gas sample from the Messinian sandstones at El-Tamad field, located near to West Dikirnis, was analysed to determine its molecular and isotopic composition. The presence of isotopically heavy δ¹³ methane, ethane and propane indicates a thermogenic origin for the gas which was cracked directly from a humic kerogen. A preliminary burial and thermal history model suggests that wet gas window maturities in the study area occur within the Jurassic succession, and the gas at El-Tamad may therefore be derived from a source rock of Jurassic age.

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