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ABSTRACT: Subsidence Related Horizontal Earth Movements in the Tia Juana Field, State of Zulia, Venezuela

Wolfgang Scherer, F. Fern'andez, M. A. Pedroza

Subsidence due to oil exploitation in the Bolivar Coast fields in western Venezuela has been monitored since 1926. Related horizontal earth movements have been measured by geodetic methods during the last 10 yr in the northern Tia Juana field area in and around the Ule tank farm and the GLP gas liquefaction plant. Damage to storage tanks and increasing appearance of fractures in the ground in the vicinity of the coastal dyke protection system warranted a detailed study by Intevep, S.A., the research and development company of Petroleos de Venezuela, S.A. PDVSA.

A surface geologic survey of the Ule area, based initially on aerophotographic interpretation of infrared photographs, flown on a 1:5000 scale, showed the area to be divided into blocks by several major faults. These faults were confirmed on the ground by visual inspection, shallow seismic refraction, geoelectric surveys and correlation of geotechnical borings.

A geodetic survey of the area with a precision of 5 mm + 2mm/km for distances, showed significant horizontal displacements of ground markers during an eight-year period. Accordingly, a measuring plan of high-precision (0.015 mm) electromechanic extensometer lines was designed for critical zones across known fault traces and fracture zones.

A conceptual model integrates geodetic and extensometer measurements of horizontal movements in the Ule area with the geologic and neotectonic frame of subsurface faults, joining systems, and tensional surface cracks. Rates of Holocene postglacial isostatic adjustments and natural, pre-industry subsidence due to degassing of reservoirs, determined by C14 radiocarbon dating, were used to calculate subsidence rates due to oil exploitation. It was found that the subsidence occurs in the area, not as previously mapped continuous function of compaction, but as discrete, fault and crack limited blocks, governed as much by reactivation of fault slippage as by subsurface compaction.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990