--> Abstract: Downdip Water Incursion and Gas Trapping Styles Along the Southwest Flank of the Piceance Basin, by G. C. Kukal; #90993 (1993).

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KUKAL, GERALD C., CER Corporation, Las Vegas, NV

ABSTRACT: Downdip Water Incursion and Gas Trapping Styles Along the Southwest Flank of the Piceance Basin

Gas is commonly found downdip of water in low-porosity and low-permeability gas reservoirs. Several authors have discussed the physical mechanism and geological characteristics of this phenomenon. However, much of the previous work that documents this style of gas entrapment has been diagrammatic. For the few cases where these diagrams are supported by actual well data, they are generally of a basin-wide scale and do not show detail of gas and water distribution.

This paper provides detailed log analysis of selected wells along the southwest flank of the Piceance basin. The study fringes the southern and southwestern margin of the basin-centered Mesaverde Group gas package from the Plateau field through the Shire Gulch, Debeque, and Logan Wash fields.

Vertical profiles of each analyzed well show the vertical distribution of gas and water through the section. Five distinct zones are identified by their interpreted interstitial fluids. The composition of these fluids with increasing depth are as follows: fresh water of meteoric origin, fresh water mixed with connate water, connate water, gas-water transition, and, finally, gas.

Cross sections show the gradations of these same zones laterally. Both the gas zone and freshwater zone occur in higher stratigraphic position and in lower structural position in the downdip direction. These relations suggest that fluid movement crosscuts stratigraphy.

The dominant migration path of both gas and fresh water appears to be lateral. The gas-water transition zone creates a temporal, dynamic reservoir seal as a result of the low effective gas permeability within this zone. However, buoyancy forces are important to updip gas migration, particularly toward the margin of the basin where gas-water contacts are common. Examples are presented of significant gas accumulations in conventional structural-stratigraphic traps along the southwest margin of the basin. It is important for exploration geologists to be aware of both of these gas trapping styles, as well as their characteristic signatures.

AAPG Search and Discovery Article #90993©1993 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah, September 12-15, 1993.