Timothy C. Hester

Organic Carbon in Bakken Formation, United States Portion of Williston Basin

The upper and lower members of the Mississippian and Devonian Bakken Formation in the United States portion of the Williston basin are black shales that are extremely rich in organic matter and are the source of much of the oil found in the basin. Organic-carbon values are calculated from formation-density logs using the equation: TOC = (154.497/^rgr) - 57.261, where TOC is organic-carbon content (wt. %) and ^rgr is formation density (g/cm3). Test calculations comparing this equation to laboratory organic-carbon analyses from 39 wells in North Dakota show an average absolute difference of 1.1% in organic-carbon content. Organic-carbon content, calculated at 159 locations in North Dakota and 107 in Montana, averages 12.1% for the upper member of the Bakken Formation and 11.5% for the lower member. There is a regional depletion of organic carbon, paralleling present-day isotherms, that reflects the conversion of organic matter to oil and subsequent expulsion of the oil from the formation. The mass of organic carbon in the Bakken Formation is approximately evenly divided between the upper and lower members, and it totals about 126 × 1012 kg in the study area, of which 102 × 1012 kg are in the thermally mature region. The assumption that 167 mg HC/g TOC have migrated out of the mature Bakken shales leads to a tentative estimate that hydrocarbons equivalent to 132 billion bbl of 43° (API gravity) oil have been expelled from the United States portion of the upper and lower members of the Bakken Formation.

Porosity of the Miami Limestone (Late Pleistocene), Lower Florida Keys

ABSTRACT This study investigates the amount and variability of porosity in the approximately 130,000-year-old politic grainstones of the Miami Limestone in the lower Florida Keys. Porosity is derived from porosimeter measurements of 326 plugs and core pieces representing 11 locations spanning about 65 km. Because larger voids in our samples result primarily from scouring by circulating water used in coring and plugging, measurements of matrix porosity, rather than of bulk porosity, represent the formation better in situ. X-ray diffraction analyses show that the rocks have undergone substantial inversion of aragonite to calcite (mean aragonite content of the matrix is 40 percent), but are not yet approaching mineralogical stabilization. Porosity of these partially stabilized rocks averages 36 percent, a decrease of 5-10 percent from that of the initial sediments. A qualitative interpretation suggests that the reduction in pore volume correlates with the degree of inversion of aragonite to calcite, and that the porosity decrease associated with complete mineralogical stabilization could be on the order of 15-20 percent. Porosity is narrowly distributed. The standard deviation of porosity for the entire data set is only 4 percent, and much of this variability is on the relatively small scale of primary sedimentary fabric. Although the pre-burial, near-surface meteoric environment may set the stage for subsequent porosity evolution, results imply that significant porosity loss and porosity diversity in similar but much older rocks develop during burial diagenesis.

Assessment of undiscovered biogenic gas resources, North-central Montana Province

In 2000 the U.S. Geological Survey (USGS) assessed the undiscovered biogenic (also known as microbial) continuous gas resource potential of the North-Central Montana Province in eastern Montana (fig. 1) as part of a national oil and gas assessment project. The assessment was based on the general geologic elements used to define a total petroleum system (TPS), including hydrocarbon source rocks (hydrocarbon generation and migra tion), reservoir rocks (sequence stratigraphy and petrophysical properties), and hydrocarbon traps (trap formation and timing). Using this geologic framework, the USGS defined the Cretaceous Judith River through Belle Fourche Biogenic Gas TPS and seven assessment units (AUs) within it, and quantitatively estimated the undiscovered continuous gas resources within each AU. Resource Summary