Abstract: Role of Diagenesis Analysis in Triassic and Paleozoic Reservoirs for Hydrocarbon Habitat Prediction in Eastern Algerian Sahara

Bekkouche, Djamel - Sonatrach

The Eastern Algerian Sahara is the most important petroleum province in Algeria. The main reservoirs are represented by the Cambrian, Ordovician, Devonian and Triassic sandstones. The primary focus of this paper is to characterize sandstone reservoirs and to predict oil and gas habitat using diagenetic tools.

The Devonian and the Triassic reservoirs are the most important oil reservoirs when more than 160 fields have been discovered containing 20% oil reserves in place. The mineralogic composition shows clean, quartzose and subarkosic sandstones for the Triassic and the Devonian reservoirs. The diagenetic history shows that the iron chlorite precipitation appears at the early stage, the quartz overgrowth development in the intermediate stage and the illitisation in the later stage.

The high reservoir quality of Triassic and Devonian sandstones (Phi 20 %, K 500 md) can be principally attributed to:

- a preservation of inherited iron clay thin rims chlorite on framework grains when they were deposited and subsequently protected quartz grain surfaces from compaction and overgrowth precipitation;

- temperature remained sufficiently low to avoid illitization and kaolinitization.

Lack of rim-chlorite allows compaction, quartz overgrowth and development of other solid cements (sideirte, dolomite) occluding pore space. That results in a drastic decrease of the preserved intregranular porosity.

Hydrocarbon bearing Devonian reservoirs sometimes present a low resistivity value (< 1 ohm) and it is difficult to precise the oil water contact and the calculation of SW. This low resistivity is generated by presence of rim iron chlorite in the sandstone. The rim-chlorite interval in the sandstone also display a good porosity in sonic log. Quartz overgrowth is the most important cement ; timing of earliest cementation started approximately 135 My at the base of Cambro-Ordovician section; but in the Triassic one, the quartz cementation occurs at 90 My. This result suggests that quartz cementation in Cambrian sandstones occurs before migration of hydrocarbons (estimated at 100 My), but in the Triassic sandstones, silica cementation began after earliest oil generation and migration. In Cambrian and Ordovician sandstones, the peak hydrocarbon generation precedes the quartz cementation and do not contribute to porosity reduction. Triassic sandstones are mostly to likely charged with oil prior to or during silica cementation. Thereby, this increases the probability of delaying quartz overgrowth development by early hydrocarbon trapping.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil