Journal of Conference Abstracts

Sedimantary Cyclicity in Peritidal Dolomite and Consequent Variations in Porous System in Jirani Dolomite (Eocene Libya)

Paola Giaj-Via¹, Paola Ronchi (paola.ronchi@agip.it)² & Teodoro Richhiuto¹

¹ ENI_LABO, Via Maritano 26, S.Donato Mil., ITALY

² ENI-PIEC, Via Fabiani, 1, S.Donato Mil., Italy

The Jirani Dolomite Formation constitutes an important target for hydrocarbon exploration in the Libyan of-shore. It represents the high stand of the lower Metlaoui Group depositional sequence, that extends from Tunisia and Libyan off-shore. The Formation has a lenticular shape and reaches the maximum thickness in C and B structures of NC41 concession. It is made up of lagoon and open lagoon limestone and dolomite limestone that grades upward to peritidal and sabkha dolomite; the peritidal facies changes laterally on the structure flanks and basinward to open shelf carbonates. Sedimentation is organised in metric shallowing upward cycles; they may be calcareous at the base in the lower part of the sequence, and grade to subtidal dolomite with an emersion diagenetic cap at the cycle top. The dolomite sedimentologic and petrographic characteristics indicate an evaporative early diagenetic dolomitization; this hypothesis is confirmed by stable isotope analyses: positive carbon values agree with a marine derived diagenesis and slightly depleted oxygen values suggest a minor recrystallization of the dolomite during burial. The early diagenesis controlled the pore system development, that later underwent little burial changes. Dolomitization caused pervasive and intercrystalline microporosity, the influx of fresh waters at the cycle top determined the formation of biomolds in the subtidal facies, dissolution vugs and microchannels in the supratidal facies. The main four microfacies that stacks from the base to the top of the ideal cycle are: intrabioclastic packstone-wackestones (L), laminated dolowackestone (D1), dolopackstones containing Mliolidae and other small bioclasts (D2) and dolowackestones with exposure traces (D3). The quantitative study of the porosity in the different facies has been done comnparing imaging analyses studies, performed on thin sections, and laboratory measures on plugs. The pore network of facies L is made of intergranular residual primary porosity and minor amounts of biomolds, its porosity and permeability mean values are the lowest among the different facies. Facies D1 has a fair amount of intercrystalline porosity but the small pore dimension gives a low permeability. Facies D2 has the best pore system: biomoldic porosity, coupled with intercrystalline porosity assure high porosity and permeability values. The diagenetic cap (facies D3) shows high porosity due to dissolution channels and vugs but the matrix is more compact than in facies D2 and thus the pore system connection is lower. On the whole the integrated petrophysical study proves that the subtidal part of the peritidal cycle (D2) has the best characteristics, due to the depositional and early diagenetic environment; whereas the strong diagenetic overprint of supratidal facies has resulted in a loss of pore connectivity. The strong link between facies, diagenesis and petrophysical properties is that lateral facies changes will produce variations of the resrvoir characteristics; for example we can expect a decrease in petrophysical values on the structure flanks and toward the margins of the depositional area. The sedimetologic study of the Jirani Dolomite has useful implications for the prediction of petrophysical changes and may help in the reservoir and exploration fields of the petroleum industry.

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