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Abstract: Structural Framework of Northern South America and Hydrocarbon Systems Analysis of Eastern Venezuela Basin: An Integrated Approach

Goodman, E. D.; L. L. Summa; M. Richardson - Exxon Production Research Co.; P. S. Koch; A. R. Green; I. O. Norton - Exxon Exploration Co.

The complex history of northern South America's margins poses special challenges for understanding hydrocarbon systems evolution. We used an integrated plate-to molecular-scale approach to begin unraveling this complex history and, in the process, validated this approach as a means of studying hydrocarbon systems in other complex tectonic settings. A summary graph relates Jurassic-present plate tectonic setting to hydrocarbon systems development in northern Venezuela. The Eastern Venezuela Basin (EVB) is part of an active, curvilinear belt of subduction, transpression and tectonic thickening, As in many foreland basins with rapid sedimentation, low-modeled heat flow values (35-40 mwatts/m2) create opportunities for maintaining active hydrocarbon charge in EVB, despite deep burial. We identified two other source rocks in EVB in addition to the well known, marine, Cenomanian-Turonian and the oil- and gas-prone coaly/paralic lower Miocene intervals. Probable Jurassic hypersaline-lacustrine and Albian marine carbonate source rocks were identified using samples of reservoired oils, seeps, organic-rich rocks and fluid inclusions. The detailed geochemistry (biomarkers, isotopes etc) of these hydrocarbons was integrated with Exxon's global database to understand the characteristics and distribution of these oils and their postulated source rocks. Our tectonic analysis suggests that the "Jurassic" oils originated in rift-controlled lakes lying at high angles to the eventual opening of the South Atlantic, and that Albian oils are related to the development of carbonate facies on the Atlantic margins. This Mesozoic rift is interpreted to be an eastward continuation of Espino Graben and to underlie both the Serrania del Interior and the Gulf of Paria, where thick evaporite sections have been penetrated. Biomarker analysis of oils from fluid inclusions, trapped in Miocene sandstones overlying the proposed graben fill, helped to constrain the distribution of the inferred, Jurassic, lacustrine source and to demonstrate the existence of effective vertical migration pathways.

Commercial hydrocarbon occurrences are concentrated in structurally-overprinted areas along the flanks of thick sedimentary accumulations. These composite depo- centers of episodic pulses of sedimentation, deposited diachronously along the northern margin of Venezuela, have created multiple maturation kitchens. Unusually efficient lateral migration was enhanced by: very rapid burial, pronounced asymmetrical subsidence patterns with long-lived focus areas, and by coarse, buried, fluvial migration "pipelines" oriented at high angles to the depocenter axes. Our thermal models suggest that tremendous volumes of hydrocarbons were lost from the mid-Tertiary "Carapita Basin" (which had blanketed the present-day Serrania del Interior) due to lack of effective traps and seals and by quenching during subsequent uplift. By mid-Miocene time, however, when source rocks from the foredeep began to mature, reservoir, migration pathways and top seal were in place, and foreland flexure resulted in normal fault traps and seals. The shifting of sedimentary depocenters with time has resulted in episodic pulses of maturation/migration in and the mixing of different oil types in EVB reservoirs and this has impacted the quality of reservoired oils. For example, oils from the terrigenous-dominated Miocene-source rocks tend to be light and sweet, whereas the marine-dominated Cretaceous sourced oils tend to be heavier and high sulfur. Mixing of the later charge, light oils with heavier Cretaceous oils has resulted in fields with variable oil qualities. Other post-emplacement processes, such as late gas migration into oil-filled reservoirs, deep burial of oil accumulations, biodegradation, and water washing have also significantly effected hydrocarbon quality.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil