--> Wamsutter “Acreage Capture:” A Case Study in Tight Gas Sand Development, GGRB, Southwestern Wyoming, USA, by G. Earl Norris, Tony Mcclain, and Debra H. Phillips; #90042 (2005)

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Wamsutter “Acreage Capture:” A Case Study in Tight Gas Sand Development, GGRB, Southwestern Wyoming, USA

G. Earl Norris, Tony Mcclain, and Debra H. Phillips
BP America Production Company, 501 WestLake Park Boulevard, Houston, TX 77079-2696, USA

BP America Production Company recently concluded a frenzy of drilling activity in the last three years of a 35 year drill-to-earn deal on the historic 1862 Union Pacific Rail Road U.S. land grant, associated with the right-of-way for the Union Pacific Railroad in southwestern Wyoming. Via a farm out in 1969, Amoco, now BP has been earning a 75% working interest in every section of land grant acreage in which it has drilled and established gas production in paying quantities. With the expiration of the deal on January 1, 2005, BP’s drilling program, which has ramped up significantly over the last three years due to strong natural gas prices, has doubled the size of the company’s asset. Approximately 230 sections and 100,000 net acres were captured. In doing so, BP’s Wamsutter Team has learned a great deal about developing reserves in these complex, paralic, tight gas sand reservoirs. In addition, in 2004, BP successfully obtained a revision of field-wide rules allowing well density to be increased to 80 acres from the previous 160-acre rule. It is believed that the ultimate spacing for optimal development could be 40 acres or less in some cases.

Along the western margin of the Cretaceous Seaway, in the area that is now the Greater Green River Basin of southwestern Wyoming, the interplay of sea-level changes and accommodation space resulted in the deposition of a diverse suite of sandstones, siltstones, shales, and coals in a complex, but predictable, sequence of tidally-dominated depositional environments. These environments of deposition ultimately control reservoir quality in sands that are prospective in a range of depths from 8,000 to 13,500 feet.

The lower Almond Formation of the Mesa Verde Group is dominated by fluvial deposition with associated levee deposits, crevasse splays, flood plain shales and some coal. The point bar sands make the best reservoirs due to reservoir quality and relative thickness compared to sandstones of other facies. This part of the section directly overlies the widespread fluvial and braided channel sands of the Ericson Formation, which typically exhibits excellent porosity and permeability, but is water filled. Fracturing and sometimes intimate stratigraphic relationships brings Ericson water up into the lower Almond reservoirs. The structural level of water encroachment is not consistent around the Wamsutter Development Area (WDA). Although the lower Almond section thickens from west to east across the area, the number of preserved channel sands decreases.

The middle Almond is more distal in the system, and deposition took place in a lower coastal plain setting with fluvial sands, tidal channels, crevasse splays, and coals. There was also a significant marine transgression during this time over much of the WDA. This event resulted in the deposition of shore face sands, ebb- and flood tide deltas, bay-head deltas, carbonaceous shales and coal. A major “line of demarcation” between coastal plain type deposits and more lagoonal dominated deposits occurs in the middle Almond and runs north south through the eastern portion of R94W.

Upper Almond sands are restricted to marine processes and are devoid of coals and other related facies of the lagoonal/coastal plain environments. Sands are dominated by long, continuous shore face bar sands, which can be correlated in some cases for many miles, quite the opposite of the lack of continuity exhibited by sands of the lower and middle Almond sections. However, within these marine bar sands, continuity can be disrupted by tidal channels and related small scale deltas owing to the tidally dominated nature of the depositional system. These changes result in wide ranges of reservoir properties, despite the good continuity of the overall sand packages. Across the WDA, these main bar sands, of which there are at least six, were deposited from east to west, back-stepping through time, as the gross package of paralic sands was transgressed by the deepening Lewis sea, marking the end of Almond deposition. These sands are probably the best reservoirs in the Almond, and include prolific production from the upper Almond in the Echo Springs-Standard Draw field, commonly referred to as the “sweet spot” by Green River Basin workers. North and east of the ES-SD field, this bar sand plunges rapidly down dip, into the Red Desert Basin where reservoir quality deteriorates leaving portions of the reservoir unproductive. However, in the upper Almond bars in the WDA reservoir properties can improve locally, usually owing to facies changes (e.g., tidal channels cutting the bar) resulting in fairly high-rate wells far down structure from poor or non-productive wells in the same bar.

The ability of these relatively low-quality sands to produce economically is at least partly a consequence of geopressures. At a depth of roughly 400 feet above the top of the Almond, this formation becomes overpressured, but gradients vary widely from nearly normally pressured (0.43 psi/ft) to as high as 0.8 psi/ft. Calculated gradients can even vary widely within the same borehole over very minimal depth changes. These circumstances are believed to be a result of disequilibrium compaction in a basin that subsided more rapidly than it could expel fluids, with some contribution to the high pressures possible by the process of hydrocarbon generation. Source of the hydrocarbons are coals and carbonaceous shales that encase most of the reservoir sands. Seismic velocity mapping has been used in parts of the field to identify local, anomalously higher pressures within the overpressured basin. Temperature gradients are also generally high across the WDA, and show significant variability. This is likely related to basin history in which a rapidly down-dropped basin is subsequently uplifted over a short period of time, and fairly recently, such that temperatures have not yet equilibrated.

In addition to production from Almond sands, a significant portion (~15%) of the total production of the area comes from submarine sandstones of the Lewis formation. As the Lewis sea ultimately transgressed the Almond Formation, a thick sequence of deeper water shale was deposited over the Almond package and can probably be considered the ultimate seal in the system. Within the dominantly marine shale, several submarine sands were deposited and, within the WDA, are considered to be base-of-slope turbidite channel and fan deposits. Significant stacked basin-floor fans also exist. New areas of Lewis production have been identified during the Acreage Capture project, particularly in the far northern reaches of the WDA and in the far southwest portions.