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
Shallow submarine geology in the Yellow and EastChina seas is dicta ted mostly by the proximity of the Yellow and Yangtze Rivers and by the late Quaternary history of the area.Most serious potential geological hazards (buried channels throughout the area, shallow gas in areas with high rates of modern sediment accumulation, and possible lateral mass displacement of both sediments) stem from these two factors.Both tectonic and growth faults are present locally, but rieither appear to present a major engineering problem.Sediment movement due to currents from tides and storms is potentially a serious problem, as is the local influence of shallow or exposed bedrock in the East China Sea.
@Copyrlght 1980 Offshore Technology ConferenceThis paper was presented at the 12th Annual OTC In Houston, Tex" May 5·8, 1980. The matorlalls subJect to correction by the author. Permission to copy Is restricted to an abstract of not more than 300 words. ABSTRACTA geotechnical and geophysical reconnaissance was performed in anticipation of offshore drilling and construction in the Orinoco River Delta, Venezuela. The survey results show that a major portion of the study area is covered by cohesive sediments with an intriguing range of properties. An examination of regional geologic history suggests that fluctuations in regional sea. level were probably the cause of sediment property variation.A relationship between undrained shear strength, water. content and liquid limit was developed, and the relationship is similar to relationships for the Mississippi Delta. The variation of liquid limit with plasticity index is expressed by a straight line relationship that is in good agreement with the data.The geotechnical and geophysical data were used for evaluating the following: anchor holding capacity, seafloor bearing capacity for guide bases, conductor pipe installation, and seafloor support of jack-up drilling rigs.
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