Influence of High-Frequency Strandline Cycles on Internal Reservoir Geometry: Point Lookout Sandstone, New Mexico and Colorado

Robyn Wright

High-frequency (< 100,000 years) transgressive-regressive strandline couplets that form basic building blocks of individual marine shoreface sandstones strongly influence strandline migration pattern and internal reservoir geometry. At reservoir scale, couplets average 7-10 m thick and are characterized by repetitive stacking of genetically related sandstone-mud rock packages. Individual cycles, or architectural elements, correlate in 3-D outcrop for at least 30 km, are locally bundled into larger discrete packages of five to eight constituent cycles, and are distinguishable in well logs.

Contrary to long-held opinion, many of these cycles are not asymmetric. Rather, they contain thin (< 3 m) transgressive systems tracts that are overlain by thicker (< 7 m) regressive systems tracts and which are separated from one another by a pronounced downdip surface. Transgressive mud rocks fine upward from siltstone to clay-shale and coarsen landward less than 10 km into thinly interbedded sandstones with abundant siderite lags. Regressive mud rocks coarsen upward and landward into prograding shoreface sandstones that may overlie transgressive sandstones in proximal positions. Landward facies transitions within transgressive and regressive components of a single cycle vary, resulting in complex lithofacies interlayering.

The significance for reservoir studies is that repeated transgressive systems tracts form predictable intraformational stratigraphic markers which may influence both horizontal and vertical fluid migration pathways. Importantly, the standard vertical profile fails to properly interpret transgressive deposits (especially sandstones) in the absence of a well-constrained lateral model.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.