AMOCO M.G. EISCHEID #1
DEEP PETROLEUM TEST CARROLL COUNTY, IOWA: PRELIMINARY
INVESTIGATIONS
R.R. Anderson (editor)
Iowa Department of Natural Resources, Geological Survey Bureau
Special Report Series No. 2, 1990, 185 p.
EXECUTIVE SUMMARY
Introduction
In 1987, Amoco Production Company drilled the M.G. Eischeid #1 petroleum exploration well northeast of the town of Halbur in west-central Carroll County, Iowa (NW¼, NW¼, SE¼, T.83N., R.35W.). The Eischeid well was Amoco's first test of the petroleum potential of the Midcontinent Rift System (MRS), a failed rift in the crust that formed about one billion years ago which extends from central Lake Superior to central Kansas. The history of the MRS is reviewed by Anderson (Review of Precambrian...," this volume) and its structure is discussed by Anderson ("Interpretation of geophysical data...," this volume). Although petroleum is unusual in rocks as old as the MRS, small amounts of oil have been collected from MRS clastic rocks (sandstones, siltstones, and shales) in the Lake Superior area. The Eischeid well reached a total depth of 17,851 feet, far surpassing the 5,305-foot depth of the previously deepest well in Iowa.
The well penetrated 2,802 feet of Phanerozoic (540 million years to present) sedimentary rocks, 14,898 feet of Proterozoic (2,500-540 million years) MRS clastic rocks, and 185 feet of Proterozoic igneous intrusive rocks. No liquid petroleum was reported during the drilling, but minor occurrences of gaseous hydrocarbons were detected. The well was subsequently plugged and abandoned. Drill cutting samples were collected during the drilling of the Eischeid well, generally at 10 foot intervals, five cores totalling 72 feet in length were drilled, and a series of down-hole logs were produced. These data were released to the Iowa Department of Natural Resources Geological Survey Bureau (GSB) in the fall of 1989 for study and the preparation of this volume. Under the direction of the GSB, a series of research investigations of these samples and data were initiated by geologists from the GSB, U.S. Geological Survey, and from the academic community. These studies were primarily directed toward evaluating the petroleum potential of the MRS clastic rocks encountered in the drilling.
Results of Investigations
Witzke (this volume) studied the stratigraphy of the rocks encountered during the drilling of the Eischeid well. The Phanerozoic section was typical of the rock strata encountered in other wells in the area. Only the basal Phanerozoic unit, the Cambrian Mt. Simon Sandstone, produced any new interpretations. Based on new information from a related study (McKay, this volume) only 20 feet of Mt. Simon strata were identified in the Eischeid well.
The underlying 813 feet of clastic sediments (initially thought to be a part on the Mt. Simon) were identified as pre-Mt. Simon. The 14,898 feet of MRS clastic rocks, informally known as "Red Clastics," is by far the thickest section of these rocks encountered in Iowa, and the thickest section known from anywhere along the trend of the MRS. Witzke (this volume) subdivided the "Red Clastics" in the Eischeid well into two informal groups, the Upper "Red Clastic" Sequence and the Lower "Red Clastic" Sequence. The groups were further subdivided into informal formations and members, and the lithologic characteristics of each unit was described. One unit in the Lower "Red Clastic" Sequence, Unit C, is dominated by dark gray to black organic-rich shales and siltstones that initially appeared to offer good source-rock potential.
Below the MRS clastic rocks, the Eischeid well penetrated 151 feet of relatively fresh, undeformed gabbro. This unit was described by Van Schmus and others (this volume) who interpreted it as a dike. They analyzed zircon crystals from the dike and used uranium-lead isotope concentrations to calculate an age of 1281 million years for the rock. This age indicates that the dike predated the formation of the MRS, and probably is an element of a suite of dikes that is widespread through North America, known collectively as the Mackenzie dike swarm.
Petrologic studies of thin-sections produced from Eischeid drill-cutting samples by Ludvigson and others (this volume) and from core samples by Barnes (this volume) were used to characterize the MRS clastic rock sequence. These studies led to the interpretation of the depositional environments of the rocks and identified the differences and similarities between the MRS clastic rocks in the Eischeid well and related units observed in their exposure area in the Lake Superior Basin.
The Proterozoic clastic rocks in the Eischeid well, especially in the Lower "Red Clastic" Sequence, were investigated for their potential to produce hydrocarbons by Palacas and others (this volume). They analyzed samples from 58 depth intervals for total organic carbon (TOC) content and selected samples for other parameters including the maximum pyrolysis temperature (Tmax), the genetic potential, hydrogen index, and the chloroform-extractable bitumens. They found that Unit C was the most organic unit, with TOC values ranging up to 1.4% and averaging 0.6%. These values are low, but many geologists consider 0.5% to be a minimum value for a rock unit to be considered a petroleum source rock. Tmax values averaged 503°C indicating that the rocks in the Eischeid well were overmature with respect to hydrocarbon generation.
This advanced stage of thermal maturity was corroborated by several other researchers. Barker (this volume) investigated fluid inclusions in calcite and quartz veins. He measured two-phase fluid inclusion homogenization temperatures of selected samples and identified two temperature populations, an earlier 200°C event and a later 140°C event. He verified the 200°C event peak by bitumen reflectance (vitrinite reflectance equivalent) measurements. Ludvigson and Spry (this volume) conducted additional measurements of two-phase and other fluid inclusions in tectonic veins. They identified temperatures ranging from 125° to 178.6° C near the top of Unit C, and reported that many inclusions are filled by methane or carbon dioxide gas. They used coordinated fluid inclusion homogenization and stable isotopic (oxygen and carbon) data to suggest that petroleum may have migrated from the axis of the rift to its outer margin. Pollastro and Finn (this volume) used clay geothermometery to calculate paleotemperatures for Unit C samples. They calculated a minimum paleotemperature of 175° to 180°C from samples near the top of Unit C, and used this information to estimate a minimum bottom hole (17,851 feet) temperature of 192° to 197° C.
Palacas and others (this volume) also calculated a genetic potential of 0.1 to 0.4 HG/g and hydrogen indices from 20 to 80 HC/g TOC for carbon-rich intervals of Unit C. They concluded that "at present, these shale beds have no potential of generating commercial petroleum...," but they suggested that significant amounts of hydrocarbon may have been generated in the geologic past.
Another important characteristic in evaluating the petroleum potential of the Eischeid "Red Clastic" rocks is their porosity. Schmoker and Palacas (this volume) studied a variety of down-hole geophysical logs to calculate the porosity of sandstone units in the "Red Clastic" rocks. They calculated porosities ranging from 1 to 6% (averaging 2.3%) within the interval from 14,450 to 17,340 feet in the Eischeid well, with 14% of that section averaging 3.5% porosity or greater. However, Ludvigson and others (this volume) and Barnes (this volume) noted that no optically-observable porosity was identified below 8000 feet. Anderson ("Review of current studies...", this volume) suggested that the porosity identified by Schmoker and Palacas might be present as microporosity or as gas and/or liquid-filled inclusions.
Conclusions
Data and samples collected during the drilling of the Amoco M.G. Eischeid #1 deep petroleum test well facilitated the preliminary division of the "Red Clastic" sequence in Iowa into two groups, the groups into eight formations, and the formations into thirteen members. The "Red Clastic" rocks in Iowa are similar in many ways to the Oronto and Bayfield groups, MRS clastic rocks exposed in the northern Wisconsin area. They appear to have similar depositional environments and probably are nearly coeval. Some differences between the lithology and petrology of the Eischeid MRS strata and those observed in the Lake Superior region can primarily be attributed to their locations in different areas of the rift.
The MRS clastic rocks encountered in the Eischeid well presently have almost no potential for producing hydrocarbons, but they have apparently generated significant volumes of petroleum at some time in the geologic past. It is possible that similar rock sequences located at a greater distance from the axis of the rift may still contain economic volumes of petroleum, but a concerted exploration effort will be required to locate such resources.
For more information contact:
Ray Anderson
phone: (319) 335-1575
e-mail: Raymond.Anderson@dnr.iowa.gov