John S. Sneider¹, Robert M. Sneider², Michael R. Todd³, George W. Bolger⁴

(1) Sneider Exploration, Houston, TX
(2) Robert M. Sneider Exploration, Inc, Houston, TX
(3) TCA Reservoir Engineering Services, Inc
(4) PetroTech Associates

ABSTRACT: Sequence Stratigraphic Controls on Flow Barrier Distribution and Quality, Cullen Field, Magallanes Basin, Chile

The Cullen field was discovered in 1954 and has produced approximately 43 mmbbls of oil and 570 bcf of gas from the Late Jurassic Springhill Formation. A sequence stratigraphic study established the reservoir and flow barrier distribution and quality to use in a mechanistic reservoir simulation study. The results of these studies helped determine the feasibility of a miscible gas injection project for the field.

The Springhill Formation was deposited within a series of progradational sequences that form an overall transgression and onlap a basement high. The older sequences in the field contain incised valleys filled with estuarine-influenced fluvial sandstones that progress upward into sequences dominated by shallow marine sandstones. Maximum flooding surfaces (MFS) and to a lesser degree parasequence boundaries form reservoir flow barriers and baffles. The distribution of the MFS is extensive; however, their effectiveness as flow barriers vary as facies change along those surfaces. This variability in flow barrier quality impacts the effectiveness of the miscible gas injection project. Injecting gas in wells with more vertical flow barriers keeps the miscible gas distributed throughout the reservoir and increases recovery.

Three major facies which have different capillary and compositional properties form flow barriers/baffles within the reservoir. Clay-rich shales that often form above the estuarine deposits have good continuity and make the best flow barriers. Calcite-cemented siltstones are good quality barriers but are not as widespread as the clay-rich shales. Calcite-cemented sandstones are effective local barriers but have a restricted distribution.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado