AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Facies-Belt Pinch-Out Relationships in a Distal, Mixed-Influence Shallow-Marine Reservoir Analogue: Lower Sego Sandstone Member, Western Colorado, USA
(1) Earth Science and Engineering, Imperial College London, London, United Kingdom.
(2) Norske Shell, Stavanger, Norway.
Lateral and proximal-to-distal facies relationships in mixed, wave- and tide-influenced shallow-marine deposits are poorly documented in both reservoirs and outcrop analogues. This study focuses on outcrops of the Lower Sego Sandstone Member, western Colorado, where tide-dominated sandstones interfinger with wave-dominated delta-front deposits within a series of stacked regressive-transgressive tongues. The stratigraphic relationships and facies-belt pinch-outs between tide- and wave-dominated deposits have been documented along continuous exposures over an area of 7 by 12 km.
Wave-dominated deposits are characterized by coarsening-upward successions with hummocky-cross stratified, very fine-grained sandstone in the upper parts. Tide-dominated facies are dominated by trough and herringbone cross-stratified, fine- to medium-grained sandstone, with intercalations of heterolithic sand-mudstone and inclined heterolithic strata. Tidal deposits are coarser grained and have a higher net-to-gross ratio, compared to wave-dominated facies, and hence have superior reservoir characteristics. Erosively based tidal deposits overlie wave-dominated lower shoreface deposits and form either (1) thick, amalgamated sand bodies, or (2) thinner sand bodies intercalated with wave-dominated deposits. Clear proximal-distal trends exist, which follow the established regional paleogeography. The proximal (western) area comprises tidal channels that erode into sub-tidal to upper inter-tidal deposits. Towards the distal (eastern) area, tide-dominated deposits form increasingly sheet-like sand bodies, which were exclusively formed in sub-tidal environments. Along the same proximal-to-distal trend, tidal sandstones decrease in thickness from 32 to 16 m over a distance of 8 km and pinch-out within another 4 km. Towards the pinch-out, tidal sandstones are increasingly reworked by waves due to successive flooding. Hence, sand body geometry reflects a combination of original tidal deposition and subsequent modification by wave reworking.
This study quantifies the intertonguing and pinch-out relationships within this mixed-influenced shallow-marine setting, providing analogue data that could improve the understanding of reservoir architecture and potential production behavior of similar reservoirs. Future work will assess the impact of these complex facies relationships, and the detail with which they are represented, on flow patterns and estimated recoveries in reservoir-simulation models.