MANSKE, M. CHARLES, OMNI Laboratories, Inc., Houston, TX
ABSTRACT: Misoa Formation Sandstone Diagenesis, Blocks III and V, Maracaibo Basin, Venezuela
The petroliferous Lower-Middle Eocene Misoa Formation of the central Maracaibo Basin, Venezuela, is considered an ideal rock unit upon which to investigate controls on reservoir diagenesis. The Misoa Formation of the central Basin is typically a fine to medium grained, moderately to well sorted sandstone, with shale interbeds, representing deposition in various delta front to shallow marine environments. Compositionally, sandstones are mainly sublitharenites and quartzarenites.
The diagenetic evolution of the Misoa Formation in Blocks III and V of the central Basin area has been established through quantitative basin modeling, and a detailed petrographic analysis involving thin section study, scanning electron microscopy, and X-ray diffraction analysis. Results show that reservoir sandstones of the Maracaibo Basin have been substantially modified by diagenesis. Significant porosity enhancement has been created in these rocks through the dissolution of unstable framework grains and carbonate cements. A major control on dissolution may be organic acid flushing prior to hydrocarbon generation; a mechanism supported by results of quantitative basin modeling (utilizing
time-temperature burial history reconstructions). A second, obvious control on porosity enhancement and reservoir quality is extensive leaching by meteoric waters (telodiagenesis) near the post-Eocene unconformity surface. Sandstones nearer this surface typically have excellent porosity and permeability, and display profuse leaching of grains, clays, carbonate cements, and even quartz.
Although Misoa sediments were affected by basin-wide tectonism including initial Eocene subsidence, an extended period of post-Eocene uplift and erosion, and subsequent Miocene subsidence, the paragenetic sequence appears uniform for the area. This diagenetic scenario reflects early burial diagenesis, telodiagenesis, and medium to late burial diagenesis stages. Authigenic minerals include various carbonate, silica, and clay phases. The clay mineral assemblage in this area is represented by common kaolinite, with subordinate discrete illite, ordered mixed-layer illite/smectite, chlorite, and expandable mixed-layer chlorite/smectite.
Reservoir quality is strongly affected by diagenetic grade, and diagenetic alteration is variable across the study area. The less altered Block V reservoir sandstones have better porosity and permeability than do coeval Block III sandstones further east. Diagenetic grade appears to be mainly a function of burial depth and temperature, including past-maximum burial depth, and proximity to the post-Eocene unconformity surface.
AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.