Chayvo Field, Sakhalin Island, Russia: Identifying a Significant Oil Leg From 3D Seismic and Execution of an Extended-Reach-Drilling Program
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 103588, "Chayvo Field, Sakhalin Island, Russia: Identification of a Significant Oil Leg From 3D Seismic and Execution of a World Class Extended-Reach-Drilling Program," by Ted G. Apotria, Randall S. Mathis, Richard J. Powell, and Bridget A. Venner, ExxonMobil, prepared for the 2006 SPE Russian Oil and Gas Technical Conference and Exhibition, Moscow, 3–6 October.
The Chayvo field was thought to contain primarily gas, given the results of drilling delineation crestal wells. 3D-seismic-amplitude anomalies indicated potential for very large, previously unrecognized oil rims. Ongoing drilling proved successful, with actual reservoir depths within a few meters of projected depths at well reaches of 9 to 11 km, underscoring the value of integrating seismic interpretation and extended-reach-well planning in a 3D-visualization environment.

Introduction
A 3D-seismic survey acquired in 1997 over the Chayvo field, offshore Sakhalin Island (Fig. 1), identified the potential for large oil rims in reservoirs that had tested mostly gas in crestal wells. The rims were identified from pairs of amplitude anomalies that conformed to structural contours and were interpreted as gas/oil and oil/water contacts. Several interpretation techniques were used to increase confidence in the estimated contacts and predict contact depths. The oil rims and contact depths were tested and confirmed with an appraisal well drilled in 2000, resulting in a significant oil project.

Exploration History
In 1971, the Chayvo anticline was identified and mapped from regional seismic data. In 1976–77, an additional 702.4 km of 24-fold 2D common-depth-point seismic lines was acquired before drilling the exploratory well. The Chayvo field was discovered in 1979 by the Russian oil company Sakhalinmorneftegas-Shelf and the Japanese consortium Sakhalin Oil and Gas Development Company.
The discovery well was drilled on the crest of the structure and penetrated a series of deltaic reservoirs containing one thin oil column and several stacked gas-pay zones. Subsequently, four additional delineation wells were drilled (1979–81), all within 200 m of the structural crest. These delineation wells also penetrated mostly gas, finding only two thin oil rims. In 1982, the field was assessed as primarily a gas resource, and development was deferred because of minimal oil resources and absence of a suitable gas market.
In 1995, a production-sharing agreement covering Chayvo and two other fields (Odoptu and Arkutun-Dagi) was awarded. In 1997, a 54-fold 3D-seismic survey (298 km2) was acquired over the Chayvo field. The new survey was used to identify and define structural elements, paleodepositional environments, and potential reservoir-fluid contacts. Interpretation revealed seismic-amplitude anomalies that conformed to structural spill and were downdip of gas accumulations seen in crestal wells indicating the potential for very large, previously unrecognized oil rims. As a result, delineation well Chayvo 6a was drilled (in 2000), which successfully tested an approximately 100-m-thick oil column in the main reservoir. The full-length paper details the identification process.