4D Time-Lapse Seismic Monitoring in the South Timbalier 295 Field, Gulf of Mexico

Abstract

Abstract We have imaged 4D seismic amplitude and impedance differences between 3D seismic surveys conducted in 1988, prior to production, and in 1994, when about half of the recoverable hydrocarbons had been produced, to attempt to quantify the variations in fluid saturations with time within turbidite reservoirs in a Gulf of Mexico oil field. The mission of this 4D seismic monitoring project was to develop the software tools and interpretation techniques 1) to isolate and interpret seismicchanges that have occurred in the field over time in terms of oil/gas/water changes in order to better understand the drainage of the field, and 2) to identify and locate bypassed pay. Two turbidite reservoirs of the South Timbalier 295 field, offshore Louisiana, Gulf of Mexico, provide contrasts in acoustic response that point to important lessons for 4D monitoring in other fields of the Gulf of Mexico. In the K-40reservoir, a "classic" updip water sweep pattern in the oil/water contact with time was deduced from the observation of the dimming of seismic amplitudes over time in this vigorous water-drive reservoir. A "finger" of dimmed seismic amplitudes that breaks updip of even the structurally highest perforations was observed between two of the producing wells. This finger may be related to sand quality and permeability pathways and/or to acoustic shadowing from brightening that occurred within the K-8/16 reservoir directly above. The K-40 sand is two-lobed, and our region growing analysis of 4D seismic changes suggests that water had migrated farther updip in the lower than in the upper lobe by 1994. In the K-8 reservoir sequence, complex depositional channeling appears to have isolated the sand from the regional aquifer system to produce a gas depletion drive reservoir. Volumetric region-growing about high seismic amplitude "seed" pointsproved very successful in mapping the turbidite fairways, but not in isolating the K-8 from the immediately underlying K-16 sand. Seismic differences between 1988 and 1994 indicate brightening over time occurred in the K-8 sand fairways, which is thought to be associated with increased exsolution of gas. A marked increase in GOR was observed between the times of the two seismic surveys in producing wells from the K-8 reservoir. Introduction The interpretation of drainage patterns and the tracking of the migration of oil, gas and water fronts in 4D, that is volumetrically with time, are valuable optimization tools for the economic development of producing oil and gas fields1. However, current production technology is predominantly 2D. Even when a 3D seismic survey is used, 2D map-based, interpretation "horizons" are derived and used by geologists, geophysicistsand, reservoir engineers, to plan and execute production decisions in virtually all currently active oil and gas fields. This process relies upon the accumulated experience-base of these integrated teams that use seismic, logging, pressure monitoring, and production histories to determine how oil, gas, and water move within reservoirs as production proceeds.