A Heavy-Oil Case Study on a Single Well in Venezuela, MFM-7S, Using Cores and Logs
Published:January 01, 1987
R. Everett, M. Herron, G. Pirie, J. Schweitzer, H. Edmundson, 1987. "A Heavy-Oil Case Study on a Single Well in Venezuela, MFM-7S, Using Cores and Logs", Exploration for Heavy Crude Oil and Natural Bitumen, Richard F. Meyer
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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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Exploration for Heavy Crude Oil and Natural Bitumen
Gross volumes of oil, which must be kept in mind to address the volume/size framework, may be thought of in order from largest to probably smallest volumes as follows: (1) generated; (2) dissipated; (3) degraded/ partially preserved; and (4) trapped and conventionally producible. Basic knowledge of these volumes may be from greatest to least in essentially reverse order.
The 332 largest known accumulations (less than 1% of the total number) account for more than three-quarters of the known 7.6 trillion bbl of oil and heavy oil or tar in more than 40,000 accumulations in the world. About 2.4 trillion bbl of estimated undiscovered conventional oil added to the known volume of 7.6 trillion bbl yields a total of 10 trillion bbl known or reasonably estimated. World-wide cumulative production of about 500 billion bbl of oil accounts for only 5% of the gross.
Oil in place must be estimated for conventional oil fields before comparison with heavy oil and tar accumulations. The size range of accumulations considered in the size distribution of the 332 largest known accumulations is from 0.8 to 1850 billion bbl of oil. The smallest conventional fields in the distribution are about 1 billion bbl because the size cut-off is 0.5 billion bbl of oil recoverable. The size distribution of the 332 largest known accumulations approaches log normal and is overwhelmed by the largest three supergiant tar deposits that hold nearly half of the total 5495 billion bbl.
Globally, the largest three accumulations, all heavy oil or tar, are in South and North America; the two largest conventional oil fields are in the Middle East. Prudhoe Bay and East Texas fields rank 18 and 34, respectively, in descending size order.