The Effects of Steam Injection In a Sandstone Reservoir (Etchegoin Formation), Buena Vista Field, California

GRANT, CHRISTIAN W., and ALANA REED,Chevron Oil Field Research Company, La Habra, CA

At Buena Vista field, California, 120 ft of post-steamflood core, spanning the middle Pliocene Wilhelm Member of the Etchegoin Formation, was taken to assess the influence of stratigraphy on light-oil steam-rock (LOSF) processes and to determine what steam-rock reactions occurred and how these affected reservoir properties.

High-quality steam [600 degrees F(300 degrees C)} had been injected(approximately 1700 psi) into mixed tidal flat and estuarine facies in an injector well located 55 ft from the cored well. Over a period of 20 months, steam rapidly channeled through a thin (approximately 7 ft ), relatively permeable (1 - 1000 md ), flaser-bedded sandstone unit. Conductive heating above this permeable unit produced, in the vicinity of the cored well, a 35-ft steam-swept zone (oil saturation = 0), overlain by a 29-ft steam-affected zone in which oil saturation had been reduced to 13%, far below the presteam saturation of 30%.

Steam-induced alteration ("artificial diagenesis") of the clay-rich reservoir rock was recognized using SEM, petrography, and X-ray diffraction. Salient dissolution effects were the complete to partial removal of siliceous microfossils (diatom frustules and sponge spicules), Fe-dolomite, volcanic rock fragments, and labile heavy minerals. Principal authigenic effects were the precipitation of zeolites (clinoptilolite and analcime), chlorite, cristobalite, and some smectite. Clinoptilolite exhibits an unusual, acicular habit, commonly "webbing" primary macropores.

The artificial diagenetic effects are first encountered in the basal 6 ft of the 29-ft steam-affected zone. Based on the distribution of the authigenic phases, we conclude that the reactions took place, or were at least initiated, in the steam condensate bank ahead of the advancing steam front. Steam-induced authigenic changes probably occurred after oil saturation had been reduced from approximately

The Songliao basin, the main oil-bearing basin of China, is a continental sedimentary one. Giant and middle oil fields had been discovered in the past 30 years, but only small gas fields have been discovered in shallow and middle depth. No giant or middle gas fields have been found.

The exploration and research work undertaken in recent years has proved that a lot of gas fields occur in the lower and deep oil- and gas-bearing formations.

The gas fields are found in two zones: a lower gas-bearing zone (Lower Cretaceous Quantou Group) and deep gas-bearing zone (Lower Cretaceous Denlouku Group and Upper Jurassic).

This paper describes the geochemistry, sedimentary structure, and the various types of gas reservoirs of Lower Cretaceous and Upper Jurassic formations; analyzes the generation, migration, accumulation of natural gas; and points out that the source rock is the black mudstone and coal bed of Upper Jurassic. The gas was coal-formed gas that migrated upward along the faults and accumulated m the sandstone of Lower Cretaceous (Quantou Group and Denloukou Group). Finally, the paper points out the prospect of deep gas in Songliao basin.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)