Petroleum Resources of the Great American Carbonate Bank (GACB): Surprising Lessons of Ellenburger, Arbuckle, Knox, Prairie du Chien and Beekmantown Reservoirs
Charles A. Sternbach
President Star Creek Energy, Houston TX
Production history. Hydrocarbon graphs, charts and maps of productive trends enable insights at the field, basin and regional scale. Approximately 3,650 fields have produced oil and gas in about 30 producing regions. Production is heavily concentrated in the US states of Texas, Oklahoma, Kansas, Nebraska, New Mexico, Michigan, Ohio, and Kentucky. More than 28,000 oil wells and 3,000 gas wells have produced 4.13 billion BO and 21.18 TCF gas cum. Most (57%) of the combined 7.66 billion BOE hydrocarbons are oil. Under current market conditions it would appear timely to review GACB reservoirs. Of note, 50 oil and gas fields with reserves of > 1MMBOE have been found since 1987, indicating discoveries in these fabled reservoirs are still occurring.
There are two giant fields greater than 500 MMBOE: Gomez (5.3 TCF) and Puckett (3.8 TCF) gas fields in Pecos County, Texas; and seven oil fields greater than 100 MMBO in Texas and Kansas. One might ask how do significant outlier discoveries like Wilburton field (400 BCF) or Maben Field (51 BCF) occur? Maben was hundreds of miles away from age equivalent production at time of its discovery. We will discuss methodologies to assess frontier areas that may yield future surprises.
Heterogeneous reservoir. Depositional settings include: 1) mid shelf, 2) deep shelf and 3) inner detrital belt. Most production comes from the mid shelf setting from dolomite reservoirs with 3-15% matrix porosity; limestone reservoirs are relatively rare. The deep shelf and inner detrital belt also produce significant hydrocarbons; field examples will be discussed.
Fractures and karst-related processes create heterogeneous reservoirs in all depositional settings. Wells can produce at spectacular rates of up to several thousand BOPD for oil and up to 50 to 100 MMCFG/D for gas. Cavernous porosity and “bit drops” routinely surprise explorers and confound petrophysical evaluation. A well with tight or impermeable rock that presents oil or gas shows may be close to unproduced oil and gas. “Dry holes” with hydrocarbon “shows” may identify nearby untapped reserves or future discoveries.
Diverse trap styles. Stratigraphic truncation traps can date from Early Middle Ordovician or subsequent periods of sub-aerial exposure and erosion. “Early” traps favor entrapment over long periods of hydrocarbon migration. Structural traps are mainly Mississippian to Pennsylvanian age, though many ages are possible. Maps and cross sections over analog fields show many and diverse trap styles.
The GACB can possess significant reservoir storage capacity. Historical production occurs where GACB reservoirs are juxtaposed with source rock and seals by the Sauk/Tippecanoe unconformity and younger unconformities (especially the pre-Woodford unconformity, eg Oklahoma City Field) and by faulting. Trap analysis can prioritize an exploration program in fault bounded structures by mapping regional thickness of sealing and non sealing strata juxtaposed with GACB reservoirs.
“Unconformity Thinking”. The Sauk/Tippecanoe unconformity: 1) creates erosional traps, 2) enhances reservoir by karst and dissolution processes (in ways not contemplated by early proponents of unconformity exploration who focused primarily on sandstone reservoirs), and 3) provides surfaces for fluid and hydrocarbon migration.
In addition to the majority of reserves found at or near the unconformity, production occurs hundreds of feet into the GACB. Generally, this occurs in tall structures where large columns of hydrocarbons fill reservoirs far below the top of GACB reservoirs. Cases of “stacked pays” in mid-shelf setting of the GACB are rare, possibly due to drilling practices. However, stacked pays can surprise and reward explorers in settings like the inner detrital belt of the Prairie du Chien group in Michigan where sandstone reservoirs interfinger with argillaceous carbonate seals.
The speaker will share insights from personal exploration experiences. It is hoped that insights from historical production, analog fields, and new tools will lead to more reserves in both old and new areas. The talk will feature highlights from the Petroleum Resources chapter in the newly published AAPG Memoir 98.
AAPG Search and Discovery Article #90164©2013 AAPG Southwest Section Meeting, Fredericksburg, Texas, April 6-10, 2013