Abstract: Stratigraphic Variations in the Paleocene and Upper Cretaceous Sands and their Effect on Gas Entrapment, Denverton Creek Gas Field, Sacramento Valley

HECTOR, SCOTT, Gary Drilling Co., Davis, CA

Denverton Creek gas field is located in the southern portion of the Sacramento Basin. The 30 Bcf field is located just west of the synclinal axis of the basin on a subtle structural high. The low relief anticlinal nose is cut by several normal faults with up to 200 ft of offset that assist in trapping the gas.

The primary reservoir is the Upper Cretaceous Bunker sand member of the Mokelumne River Formation. It is gas productive near its "0" edge where it is truncated by the Paleocene unconformity. Gas that has migrated to the zero edge of the sand is trapped by underlying and overlying shales. Faults also assist the trap geometry.

A secondary but significant pay in the field is the Paleogene Anderson sand. Pay in this zone is limited to the western edge of the field. The primary trap mechanism is structural with pools located next to the Lambie fault, a normal fault that typically cuts out 200 to 300 ft of section and forms the western edge of the field. Anderson gas pools occur both in "horst" blocks where the Lambie fault intersects smaller faults and in fault-closed bowings against the Lambie fault itself. However, stratigraphic variations in the various sands within the Anderson section assist in trapping gas. Variations in the shallow marine to non-marine section have led to significant pay intervals at the top, middle and base of the sand interval. However, these same variations have made development difficult, as evidenced by numerous dry holes within the outlines of the field. These complexities have also allowed for recent discoveries in the 30-year old field.

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California