--> Abstract: Sequence Stratigraphy and Turbidite Reservoir Characterization of Biogenic Gas Fields in the Guadalquivir-Gulf of Cadiz Miocene Basins, SW Spain, by W. Martínez del Olmo, S. Torrescusa, C. Riaza, and C. García; #90933 (1998).

Datapages, Inc.Print this page

Abstract: Sequence Stratigraphy and Turbidite Reservoir Characterization of Biogenic Gas Fields in the Guadalquivir-Gulf of Cadiz Miocene Basins, SW Spain

Martínez del Olmo, W.; Torrescusa, S.; Riaza, C., García, C. - Repsol Exploración

The Guadalquivir-Gulf of Cadiz Basin is a long and narrow marine foreland basin developed during Miocene times between the northern passive margin (Hercynian Meseta) and the southern fold and thrust belt (Alpine Betic mountains).

Two main Miocene sand-shale turbidite episodes have been identified. The first one is a channel-levee and lobe facies system within the HST of the Upper Tortonian (Bética) Depositional Sequence developed in the onshore region. The sedimentation area and the internal architecture were related to a syntectonic longitudinal axis of the relative shallow, marine basin. The fast progradational turbidite system characterize a very broad area of sedimentation with frequent thin to medium layers (2 to 15 m) of sandy reservoirs. Since 1982 fifteen commercial small gas fields have been discovered. All the gas accumulations have a stratigraphic trap control: small vertical closures generated by the sedimentary morphologies at the top of the levees and proximal lobes geometry.

The second sand-shale turbidite system corresponds to a Slope and Basin Floor Fan complexes within the LST of the Late Messinian-Early Pliocene (Marismas) Depositional Sequence, developed in the offshore region. This system was fed by a long and narrow incised valley (150*10 Km), well controlled by seismic along the onshore region, eroded during the late Messinian sea level fall. Its internal architecture is in response to several events of active sediment supply of channeled and non-channeled high-density turbidite currents. The porosity and thickness of the reservoir decrease towards the distal facies of each individual sand depositional event. Outside of the distal positions many wells tested up to 40 m of net sand reservoirs with porosities ranging from 15 to 30%. Since 1980 seven gas fields have been discovered in this turbidite reservoir system, only the biggest one having a structural trap control.

In the whole basin of 12000 Km2, a number of 101 exploratory wells have been drilled up to date. 45 of them were completed before 1980 without success. Since then, using more modern ideas and techniques, 56 wells have been drilled with the following results: 22 of them encountered hydrocarbons, but only 17 (15 onshore & 2 offshore) have been developed at the moment with sizes ranging from 2 to 50 Bcf.

Both turbidite systems, especially the late Messinian SFC & BFFC, in the offshore Atlantic Ocean margin, represent a large stratigraphic trap system with good reservoir and sealing conditions. The small size of gas discoveries is the result of the limited efficiency of the biogenic gas prone source rock in the basin.

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