Abstract: Depositional Patterns and Diagenetic Sequences in Cretaceous Cogollo Group, Maracaibo Platform, Venezuela

T. J. A. Reijers, P. Bartok

In the northern part of the Maracaibo platform the middle Cretaceous Cogollo Group consists predominantly of carbonate rocks which generally reflect a shallow to very shallow marine depositional environment, which commonly is restricted and slightly euxinic. Locally, the carbonate rocks have been replaced by glauconite and quartz sands in varying degrees.

Two carbonate-rock types are present, each with a specific depositional character and a typical diagenetic overprint, and exhibit varying amounts of texture-related porosity. (1) Grainstone bars are composed mainly of skeletal fragments and coated grains, which underwent some leaching. To a great extent, however, the depositional and leaching porosity subsequently was destroyed by early overgrowth cements, mainly around echinoid fragments, and drusy and blocky cements. (2) Pelecypod biostromes are made up of moderate amounts of caprinid rudists and larger quantities of oysterlike pelecypods. Remnant porosity in the rudists is in some cases the result of incomplete cementation of leached skeletal fragments, or of body cavities.

Another carbonate-rock type with potential reservoir qualities is dolomite which forms a crystal fabric with remnant intercrystalline space. Apparently the dolomite is preferentially in the lower half of the Cogollo sections. A full investigation by means of isotopic studies as to the true nature (meteoric, brackish, salt or hydrothermal water) and time of formation (early or late in the diagenetic sequence) of dolomite shows that, in the Maracaibo platform, 12 data points allow a valid approximation of the regional distribution of carbonate-rock types with different degrees of reservoir potential.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC