GHOSH, SANTOSH K., Petroscan I.P. Consultants, Calgary, Alberta, Canada

ABSTRACT: Reservoir Quality in Relation to Facies, Burial History, and Oil Migration: A Review

The diagenetic make-up and, in consequence, the ultimate reservoir quality of a sand body depends on depositional facies, as well as on burial history, diagenetic fluid chemistry and timing of oil migration. A regional subsurface study in the Maracaibo Basin, Venezuela along a northwest - southeast transect in the Urdaneta, Lagunillas and Barua-Motatan fields reveal that depositional facies of the reservoir sandstones range from mainly fluvial (Urdaneta) to deltaic-coastal marine in Lagunillas, and typically sublittoral in the Barua-Motatan area. Factors such as granulometry, development of early pore-lining chlorite (in marginal marine facies), infiltrated grain-coating clays (fluvial B-6 sands), and proximity of reservoirs to the post-Eocene unconformity, have preserved, enhanced or otherwise influenced reservoir quality.

Data on authigenic mineralogy, vitrinite reflectance and burial history indicate a progressively greater subsidence from northwest to southeast. As a result, diagenetic grade is immature in Urdaneta where porosity/ permeability values are the highest. In contrast, the Barua-Motatan field in the southeast is typified by a higher grade of diagenesis, varied authigenic mineralogy, and severe deterioration of reservoir quality. Similarly, degree of compaction progressively increases from northwest to southeast.

Timing of oil migration have parallelly contributed to systematic regional variation in reservoir quality. An early migration in Urdaneta has been favorable to preservation of original porosity. In contrast, late migration during Upper Miocene, in the Barua-Motatan field, indirectly permitted abundant silicification and consequent destruction of virtually all primary porosity. Thus both pre- and post-depositional parameters are important constraints which should be considered while evaluating quality of siliciclastic reservoirs.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.