Lower Cretaceous Cogollo Group, Maracaibo Basin, Venezuela: Sedimentology, Diagenesis, and Petrophysics

The Cogollo Group, consisting mainly of carbonates, was deposited over a broad and irregular stable shelf. High-energy, shallow, normal-marine environments are represented by grainstone/packstone bars, interbar deposits, and pelecypod biostromes. Low-energy, restricted, shallow-marine environments are shown by back-bar to tidal-flat deposits including lagoonal sediments rich in organic matter, shales, and glauconitic and quartzitic sandstones. On the basis of lithologic markers the Cogollo Group, in central Lake Maracaibo, is subdivided into a series of rock units which indicate two major lithologic-environmental subdivisions. The lower Cogollo is characterized by a combination of carbonates and siliciclastics derived from the Guayana shield and associated highs. The upper Cogollo is represented by cleaner carbonates deposited under higher energy conditions. Grainstone/packstone bars are common together with pelecypod biostromes and muddy lagoonal sediments. Cogollo carbonate production rates kept pace with subsidence and therefore are mostly a "build-up" type. Diagenetic models describing the porosity history of the grainstone bars and pelecypod biostromes, when superimposed on the depositional model, indicate carbonate fabric units. Processes which destroyed reservoir space (mainly cementation) were important, hence, present porosity and permeability values are low (^Thgr <= 12%; K <= 3.2 md). Siliciclastic and dolomitic intervals have some reservoir potential. Detailed correlation between characteristics of the carbonate fabric units and well log expressions is hampered by the complex mineral composition and generally poor texture-related porosity and permeability properties of the units. Core log correlations of the Cogollo Group show three broad subdivisions: (1) shales/marls, (2) dirty (ductile) carbonates/sandstones, and (3) clean (brittle) carbonates/sandstones, in order of increasing susceptibility to fracturing. This "petrophysical" subdivision in combination with the environmental model may be of value in future regional studies aimed at tracing rock units with matrix and/or fracture-related reservoir potential within the Cogollo Group. Diagenetic processes were ongoing during hydrocarbon migration. Thus the modest texture-related reservoir space in the carbonates was, and still is, able to store the oil. Local fractures and leached intervals in the carbonates allow oil to drain and be produced. Thus, fractures form a significant element in Cogollo production capacity.