Field—Analogue for Ordovician Exploration in the
Enzo Insalaco2, Frédéric
Walgenwitz2, Jean-Pierre Houzay2 (1) Total E&P USA,
Maben Field produces gas from the Ordovician in the
Maben Field rreservoir quality is clearly influenced by complex interactions between depositional facies, dolomite texture/crystal size, hydrocarbon migration/alteration and diagenesis. The depositional system was low energy with a scarcity of sabkha and other evaporites. Not all grainstones are porous reflecting porosity occlusion by calcite, limpid dolomite, bitumen in order of importance. The diagenetic history of the Ordovician reservoir is described as early replacement dolomitization (tidal flat), followed by limpid dolomite and calcite type 1 cementation. Dolomite texture is controlled by the sedimentary facies: sucrosic, porous dolomite in grainstones, anhedral tight mosaics in mudstones. Dissolution may be associated with exposure surfaces and meteoric invasion. Late deep burial dolomitization is at a low level and does not strongly alter the poroperm template inherited from the early stages.
The Mississippian Neal Shale is the primary source rock. The average TOC, reconstructed, is estimated to be 4.5% with a corresponding HI of 400 - 600. The kerogen type is 90% amorphous kerogen and 10% algal kerogen. High reflectivity of bitumen (3.65%) indicates high thermal stress acquired by deep burial during Late Paleozoic. Remnants of very high maturity oil in bitumen strongly suggest a pre-existing oil accumulation. Nonsoluble high maturity bitumen + high maturity oil remnants, suggests oil-cracking rather than simple precipitation (i.e. de-asphalting). Gas is almost pure methane which isotopically suggests very high maturity.