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Abstract

Planning for optimum production from heavy-oil reservoirs requires a deep understanding of their geological and petrophysical characteristics, synthesized from intermittent core and continuous log data. In this paper the study developed for the MFM-7S well, operated by S. A. Meneven in the Faja Petrolifera Del Orinoco area of eastern Venezuela, is described.

The case study is one of several petrophysical research projects carried out by INTEVEP S.A., the research and development affiliate of Petroleos de Venezuela, either on its own or in cooperation with other companies. These projects are part of INTEVEP's Petrophysical Research Program for the Orinoco Heavy Oil Belt (Faja Petrolifera del Orinoco).

The well is located near the south-central section of the east-west-trending heavy-oil belt located immediately north of the Orinoco River. Twenty-two log surveys and extensive whole and sidewall core analyses provided an extremely complete data set. Key logging surveys included the enhanced resolution log, the gamma spectrometry log, the natural gamma log, the nuclear magnetic log, the electromagnetic propagation log, the stratigraphic high resolution dipmeter log, the dual laterolog, and the microspherically focused log.

The Oficina Formation, in this well, is a complex series of stacked, river-derived, heavy-oil saturated, sand- dominated sediments overlain by shale-dominated estuarine (brackish) sediments, all of which unconformably overlie the Precambrian basement. For this environment, the following provided the data necessary for devel-oping the production strategy. The sand units, as well as the thickness of the sand and shale strata were determined. Paleontological, geochemical, and mineralogical core analyses, in addition to geochemical log measurements, helped distinguish between estuarine and fluvial deposition. Grain size and sorting, as well as dipmeter-defined bedding and geochemical measurements, were instrumental in describing the crevasse splays and channel fills. The higher water content of the fine-grained crevasse splay sediments correlated with the response difference between the nuclear magnetic and electromagnetic logs. Factor analysis techniques, applied to the elemental analysis, mineralogical analysis, and other rock properties of the 124 sidewall cores, provided a geochemical definition of the reservoir. The geochemical definition permitted a quantitative description of the mineralogy including clay types throughout the reservoir. Finally, the vanadium concentration in the hydrocarbon, obtained from core and log measurements, was used to determine the API gravity of the oil in each sand unit.

The complete data base created from the suites of log and core measurements made in the Meneven well allowed us to employ new and innovative interpretation methods to resolve the geological and geochemical environment, and characterize the heavy oil in situ, for each sand unit.

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