AVO in the Late Cretaceous M1 Sandstone, Eden Yuturi Field, Ecuador
Nicholas K Boyd, Mario Cardenas, and Milton Galarraga
The Eden Yuturi field produces from five sands in the late Cretaceous Napo formation of Ecuador’s Oriente Basin. The productive wells are located on the top of structure in a conventional play; fault bounded on the west side and with a water leg confining the remaining sides of the formation. Based on 2-D seismic data, initial drilling in 1970 yielded limited oil shows in three of the five sands, while a second exploratory well in 1996 produced significant oil shows in all five. The challenge for the 190 km2 3D seismic data is to delineate the water leg, and more importantly to generate new leads for further exploration. Seismic interpretation techniques beyond structural and stratigraphic mapping have been tried, but are not common practice, in Oriente Basin exploration. The application of amplitude-versus-offset (AVO) techniques can provide new interpretational insights, specifically into pore-fluid and lithology prediction. Integral to the AVO process is the analysis of well-based offset synthetics and employing Gassman fluid substitution. To uniquely perform these analyses requires compressional, and shear data which are acquired from full waveform or dipole sonic tools. The independent shear data not only provides for the creation of offset-seismic synthetics, but also for a regression equation of Vs from Vp. This regression allows the computation of synthetic shear logs in wells with only compressional data and therefore offset synthetics at these locations. The comparison between the seismic and the reservoir response via offset synthetics is an important constraint on the interpretation of the AVO data.
The M1 sand was chosen as an initial test of the applicability of AVO techniques in the Napo at Eden Yuturi,. The M1 is the youngest of the five producing sands in the field and is its second largest producer. In the Oriente Basin, the M1 has proved to be a highly productive reservoir and at Eden Yuturi it presents an attractive target for further exploration and development. More than 90 wells are located in the field with 20 wells producing 16,000 BOPD from the M1. Importantly for the AVO analysis, 12 of these wells have dipole or full-waveform sonic logs. In the Eden Yuturi M1 reservoir study, synthetics from 35 wells were studied with pore fluid ranging from oil (200 API) to water (14,000 ppm salinity) and production as high as 2,000 BOPD. In all cases the analyses of the offset synthetics indicate a clear and consistent oil-water AVO separation and comparison with the seismic data indicates a high correlation between AVO anomalies and known production. Proceeding from the testing and verification at the known wells into an exploration-reconnaissance mode employing seismic data only, the AVO analysis indicates at least two untested anomalies, one of which is of significant size. These anomalies are the leads for new exploration in the Eden Yuturi field.
Nick Boyd started in the industry as a seismic computer (1981) and later assistant party manager on a Petty-Ray Geophysical seismic crew. Convinced that his future lay in geophysics, he obtained his BA (1986) in Geophysics from the University of California, Berkeley and his Ph.D. (1993) in Geophysics from the University of Wyoming. After finishing his Ph.D. he started as a post-doctoral researcher and later research scientist at the Institute for Energy Research, University of Wyoming. His research concentrated on the seismic identification and mapping of abnormally-pressured, deep-basin gas reservoirs using velocity and AVO workflows. In 2001, he helped start Innovative Discovery Technologies, and as senior geophysicist continued deep-basin gas exploration in the Rockies and around the world. In 2003, he started his own consulting practice offering geophysical workflows and onsite troubleshooting. In 2008, he joined Halliburton Consulting as a senior managing consultant concentrating on geophysics workflows in AVO, pore-pressure prediction, seismic attributes, and prestack processing/imaging. He is a member of SEG, AAPG, EAGE, AGU, and GSA.