On the basis of lithology and sedimentary structures, six sedimentary facies (A-F) were distinguished in cores of Shannon Sandstone in Hartzog Draw field, Campbell County, Wyoming. Core porosities and permeabilities are highest in facies A (average 15% and 16 md), moderately high in facies B (average 13% and 11 md), low in facies C (average 11.3% and 3.5 md), and very low in facies D and E (average 9-10% and 1-2 md). Dispersed detrital clay is a primary control on porosity, but in cross-bedded sandstones of facies A and B, the main productive interval, dispersed detrital clay is sparse and variations in porosity are due mainly to diagenesis.

Microporosity does not contribute to permeability. It is low in facies A and B, high in facies C-E, and is associated mainly with detrital clay. As microporosity increases, water saturation increases.

The paragenetic sequence inferred is: (1) siderite cementation; (2) formation of a brown, grain-coating clay, possibly illite/smectite; (3) quartz cementation; (4) calcite and dolomite cementation; (5) dissolution of carbonate cements and some framework grains; (6) a second generation of quartz cementation along with some chlorite and interlayered illite/smectite formation; and (7) oil emplacement.