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The AAPG/Datapages Combined Publications Database

Rocky Mountain Association of Geologists

Abstract


The Paradox Basin Revisited – New Developments in Petroleum Systems and Basin Analysis, 2009
Pages 604-633

New Techniques for New Discoveries–Results from the Lisbon Field Area, Paradox Basin, Utah

David M. Seneshen, Thomas C. Chidsey, Jr., Craig D. Morgan, Michael D. Vanden Berg

Abstract

Exploration for Mississippian Leadville Limestone-hosted hydrocarbon reservoirs in the Paradox Basin is high risk in terms of cost and low documented success rates (~10% based on drilling history). Only 100 wells have penetrated the Leadville over an area of 7500 mi2 (19,400 km2), which equates to about one well per township. The potential for more hydrocarbon reserves is thus enormous, but the high cost of three-dimensional (3-D) seismic exploration methods in environmentally sensitive areas deters small independents from exploring for Leadville hydrocarbon reservoirs.

This study was therefore initiated to evaluate the effectiveness of low-cost, innovative, noninvasive, surface geochemical methods for predicting the presence of underlying Leadville hydrocarbon reservoirs. Lisbon Field, San Juan County, Utah, was chosen for testing because it is the largest Leadville oil and gas producer in the Paradox Basin, and a nearby Leadville field (Lightning Draw Southeast) with nearly virgin reservoir pressure is also available for comparison. In comparison with Lisbon Field, Lightning Draw Southeast Field, San Juan County, Utah, is smaller, with more nitrogen and helium, and has productive intervals in the overlying Ismay zone of the Pennsylvanian Paradox Formation.

The main conclusion of this study is that certain surface geochemical methods can discriminate between productive and non-productive subsurface reservoir areas. Variables in surface soils that best distinguish productive and non-productive areas are light (mainly ethane and n-butane) alkane and heavy (C30+) aromatic hydrocarbons. The volatile and liquid hydrocarbons presumably ascend to surface along faults within and at the margins of the fields. Discriminant functions developed for both fields predict one another, suggesting a compositional link between Lisbon and Lightning Draw Southeast. Mercury and lead are indirect indicators of hydrocarbon microseepage and they could be derived from the oil itself. Helium and carbon dioxide anomalies in free-soil gas at the margins of Leadville reservoirs are the most diagnostic indicators of underlying Leadville reservoirs. These gases are enriched in Leadville reservoirs as compared with overlying productive intervals of the Ismay zone of the Paradox Formation. Anomalous hydrocarbons, carbon dioxide, and hydrogen in free gas over the Lightning Draw Southeast Field may be derived from productive intervals within the Ismay zone, Leadville Limestone, or a combination of both reservoirs.

Cost-effective geochemical exploration for hydrocarbon-charged Leadville Limestone reservoirs would be staged as follows:

1. Regional geochemical exploration - collection of soil samples at large (>1,640 ft [>500 m]) intervals for thermally desorbed (C1-C12) and solvent-extractable (C6-C40) hydrocarbons analysis.

2. Detailed geochemical exploration - follow-up of regional hydrocarbon anomalies with collection of deep free-gas samples at short (<300 ft [<100 m]) intervals for analysis of hydrocarbons (e.g. propane) and fixed gases (e.g. helium and carbon dioxide) that are diagnostic surface indicators of Leadville Limestone reservoirs based on this study.

Areas with anomalous concentrations of propane, helium and carbon dioxide in free gas samples could then be followed up with 3D seismic to better define drillable structures.


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