Patrick Draw Field, Sweetwater County, Wyoming--An Old Stratigraphic Trap: ABSTRACT
The search for new petroleum reserves can be greatly implemented by a more thorough understanding of why petroleum is trapped where it is. The Patrick Draw field, discovered in 1959, started a wave of exploration effort in the Rocky Mountains area to find additional giant stratigraphic traps in the Upper Cretaceous rocks where porous and permeable sandstones pinch out on structural noses. The failure to find another Patrick End_Page 1574------------------------------ Draw, despite a widespread exploration effort, signifies that factors not generally considered must have had a dominant influence in the accumulation. These factors are revealed only by examining the geologic history of the area, beginning with the deposition of the reservoir and source rocks and studying the structural attitude of these rocks through time. Although several sandstones are petroleum-productive at Patrick Draw, the principal producing zone consists of two sandstone bars at the top of the Almond Formation (Upper Cretaceous). The spatial dimensions, lithologic characteristics, and stratigraphic framework of these bars suggest that they are barrier bar sandstones deposited along the margin of the Lewis sea. These porous and permeable linear barrier bars have a general north-south trend and, updip to the west, grade into impermeable shale and sandstone that were deposited in a swamp and lagoonal environment. A second important productive zone occurs approximately 40 ft. below the top of the Almond Formation. The areal distribution, lithologic nature, and stratigraphic framework of sandstones in this zone suggest that they were deposited as parts of a tidal delta in a lagoon. Each of the three main productive sandstones has a different oil-water contact. The geologic history of the Patrick Draw area shows that, by the beginning of the time of deposition of the Lance Formation (Upper Cretaceous), conditions were favorable for petroleum accumulation. The reservoir sandstones had 1,200 ft. of overburden and several million years had elapsed since the reservoir sandstones were deposited. An early trap was formed where these sandstones 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 first trap was opened and the accumulation spilled northward to be trapped at the present location of Patrick 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, the structural modification of the trap through time, and the associated origin and migration problems are hidden factors that play the dominate role in formation of a large petroleum accumulation. Exploration geologists must know more about the regional framework of sedimentation and the cause and effect of incipient structural development in depositional areas, and understand how these factors relate to the geologic history of a region. End_of_Article - Last_Page 1575------------