Geologic Hazards Related to Offshore Drilling and Construction in the Orinoco River Delta of Venezuela

Abstract

This paper describes the results of a seabed and shallow stratigraphy reconaissance survey conducted offshore the Orinoco River delta in Venezuela. Several geologic features were encountered that may be hazardous to offshore drilling and construction. Introduction Since Dec. 1977, the Centro de Investigacion y Desarrollo (INTEVEP S.A.) of Petroleos de Venezuela S.A. (PDVSA) has been conducting a general geotechnical investigation in the Orinoco River delta region in cooperation with McClelland Engineers Inc.To assess the safety of oil drilling operations, this study has been carried out with these main objectives:define the basic geotechnical and geologic characters of the surficial sediments,identify the potential geologic hazards for drilling and future oil production, andgain an understanding of the sedimentary process and the recent geologic history of the area. A high-resolution geophysical survey and a sampling program were conducted in the investigation area offshore the Orinoco River delta. A wide range of geotechnical, geologic, and chemical analyses were performed on the samples. As a base for the general understanding of the area of interest, a broad geologic study with emphasis on tectonics was carried out on a regional scale. Methods of Study The explored area covers 12,500 km of the continental shelf and the continental slope offshore the Orinoco River delta. The maximum water depth is 600 m, which is reached only in the northeastern part of the investigated area. Ninety percent of the geophysical lines were run in water depths between 10 and 200 m, the lowest limit corresponding to the limitations of the survey vessel (Fig. 1). Almost no investigation had been carried out before -in this region, and only limited data could help to define the geophysical grid. For this reason, it was decided to begin with a broad reconnaissance phase over a 10 × 10 km grid. These profiles provided general information that was used to define the most interesting areas that should be covered by a 5 × 5 km grid. Then, according to the data obtained, two main areas of high geologic complexity that seemed to be most hazardous to petroleum exploration were identified and surveyed with a 2.5 × 2.5 km grid.Accurate positioning was assured using an Argo DM 54(TM) range-range phase comparison system, with three fixed shore stations and a mobil station aboard the vessel. Generally, the accuracy of the system can be considered in the order of 10 m. Signal instability and loss of signals sometimes occurred at night, but only at great distances from the shore stations. Coarse checks were made periodically with a satellite system. JPT P. 764