abstract

Most of the gas production in this region comes from earlier shallow zones (<2,000 ft) in the lower Strawn or from deep zones (4,000-6,000 ft) in the Atoka to Marble Falls. The reservoirs, except for minor production from carbonates, are in clastic strata ranging from conglomerate to fine sandstone that are sporadic, lensing bodies of limited areal extent. The production is from clastics that occur in an eastward-thickening sequence lying just west of the Ouachita foldbelt.

The Aledo Southeast 1,200-ft Strawn field is one of the most profitable gas fields in the area. The marked linearity of this field demonstrates that the productive sand body is of fluvial origin. Its general east-west orientation indicates a fluvial system that flowed westward from the Ouachita highlands, and its flared shape on the western end suggests shoreline redistribution of the sands. Study of core and cuttings clearly reveals fluvial features, such as mud clasts, ripple laminations, wood fragments, and slump structures.

Detailed mineralogic analyses by x-ray diffraction and scanning electron microscopy indicate the shallow and deep reservoirs are feldspathic sandstones that are variously limy or dolomitic and contain major amounts of kaolinite and lesser amounts of illite, chlorite, and mixed layer clays.

Isopach maps combined with structure contour maps show the field is a trap formed where an east-west channel sand is tilted downward on the north side intercepting a gas-water contact. The south (updip) margin is a sand pinch-out. A marked sag, caused by differential subsidence, intercepts the gas-water contact and interrupts continuous production along the channel beneath the Parker-Tarrant county line.

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