Time-Stratigraphic Analysis and Petroleum Accumulations, Patrick Draw Field, Sweetwater County, Wyoming

Abstract

The search for new petroleum reserves can be implemented greatly by a more thorough understanding of why petroleum is trapped where it is. The Draw field, discovered in 1959, started a wave of exploratory effort in the Rocky Mountain area to find additional giant stratigraphic traps in the Upper Cretaceous rocks where porous and permeable sandstone pinches out on structural noses. The lack of success in finding another Patrick despite this widespread exploratory effort, means that factors not generally considered must have had a dominant influence in the accumulation. These factors are revealed only by reconstructing the geologic history of the area, beginning with the deposition of the reservoir and source rocks and tracing the structural attitude of thes rocks through time. Although several sandstone reservoirs produce petroleum at Draw, the principal productive interval consists of two sandstone bars at the top of the Almond Formation (Upper Cretaceous). The spatial dimensions, lithologic character, and stratigraphic framework of these bars suggest that they are barrier-bar sandstone bodies deposited along the margin of the Lewis sea. These porous and permeable linear barrier bars have a general north-south trend and grade updip on the west into impermeable shale and sandstone that were deposited in a swamp and lagoonal environment. A second important productive interval is approximately 40 ft. below the top of the Almond Formation. The areal distribution, lithologic character, and stratigraphic framework of sandstone in this interval suggest th t it was deposited as a tidal delta in a lagoon. Each of the three main productive sandstone bodies has a different oil-water contact. The geologic history of the Draw area shows that, by the beginning of deposition of the Lance Formation (Upper Cretaceous), conditions were favorable for petroleum accumulation. The reservoir sandstone beds had 1,200 ft. of overburden which had accumulated in the several million years since the reservoir sandstone beds were deposited. An early trap was formed where these sandstone beds were warped over an east-plunging structural nose, and early migration of petroleum produced a large accumulation a few miles south of the present field. When the present Wamsutter arch came into existence in post-early Eocene time, the old trap was opened and the accumulation spilled northward to be trapped at the present location of the Draw field. The search for more Patrick Draws must include more than an analysis of present structure and potential reservoir rock. The time of formation of the trap, structural modification of the trap through time, and associated origin and migration problems are hidden factors that play the dominant roles in the formation of a large petroleum accumulation. Exploration geologists must learn more about the regional framework of sedimentation--the cause and effect of incipient structural development in depositional areas. They must understand how these factors relate to the geologic history of a region.