Heterogeneities of a Low Permeability Exhumed Petroleum Reservoir, El Abra Formation, Sierra el Abra, NE Mexico

Sean T. Brennan

Characterization of heterogeneities in low permeability petroleum reservoirs is typically problematic, mostly due to the lack of research on three dimensional reservoir analogs. In the Sierra el Abra of northeastern Mexico here is an exhumed petroleum reservoir exposing mid-Cretaceous Aba limestone. This unit is the reservoir for the famous Golden Lane fields of northeast Mexico.

This study focused on three-dimensional exposures in one quarry of the Sierra el Abra, allowing analysis of a stratigraphic thickness of fifty meters differentially oil stained restricted platform limestone. Preliminary petrograph observations of fluid inclusions indicate that clear UV-fluorescent petroleum, brown non-fluorescent petroleum and gas condensate were present in the reservoir. The fluid inclusion observations coupled with the pervasive staining in outcrop indicate that the reservoir was once charged with petroleum.

Based on permeability analyses, permeability is virtually non-existent (less than 0.001 millidarcy) for all units of the shoaling upward packages which constitute the reservoir. However, porosity analyses indicate that there is tratigraphic control on the heterogeneity of the reservoir system. Within the shoaling upward packages, the peloidal phases have the highest porosities. Based on field observations, the peloid rich units had the darkest oil staining of all units within the quarry. Therefore, microporosity within the peloids was the major conduit for the charging of this reservoir.

This exposed reservoir analog provides relevant data for the recognition of heterogeneities within reservoirs as well as understanding of low permeability reservoirs and their production potential.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California