Changes

However, the Niobrara Formation includes not only good organic-rich mudstones, but also interbedded organic-lean carbonates, typically referred to as the Niobrara A, B, and C carbonates, with the B carbonate being the primary production zone.

However, the Niobrara Formation includes not only good organic-rich mudstones, but also interbedded organic-lean carbonates, typically referred to as the Niobrara A, B, and C carbonates, with the B carbonate being the primary production zone.

Data from core chips of the Golden Buckeye Petroleum 2-Gill Land Associates well demonstrate this vastly different TOC content (Tanck, 1997) (Figure 8). This well flowed 20.7 m3 (130 bbl/day) of oil and 7220.8 m3 (255 mcf/day) of gas, with only 1.11 m3 (7 bbl/day) of water from the Niobrara B interval. The GOR is reported at 308.1 m3/m3 (1730 scf/bbl). The thesis by Tanck (1997) did not include carbonate carbon data, although calcite content is reported to be 84% at 2066.2 m (6779 ft). The productive Niobrara B is found in the 2054.3 to 2065.0 m (6740 to 6775 ft) interval, where oil crossover exists (Figure 8). Oil saturations range from 63 to 80% of pore volume, with porosities of approximately 5 to 6% in this interval (Tanck, 1997). A deeper zone at 2075.7 to 2080.3 m (6810–6825 ft) has similarly high oil saturations, but much lower porosities in the 3 to 4% range (Tanck, 1997).

Data from core chips of the Golden Buckeye Petroleum 2-Gill Land Associates well demonstrate this vastly different TOC content (Tanck, 1997) ([[:File:M97Ch1.2FG8.jpg|Figure 8]]). This well flowed 20.7 m3 (130 bbl/day) of oil and 7220.8 m3 (255 mcf/day) of gas, with only 1.11 m3 (7 bbl/day) of water from the Niobrara B interval. The GOR is reported at 308.1 m3/m3 (1730 scf/bbl). The thesis by Tanck (1997) did not include carbonate carbon data, although calcite content is reported to be 84% at 2066.2 m (6779 ft). The productive Niobrara B is found in the 2054.3 to 2065.0 m (6740 to 6775 ft) interval, where oil crossover exists ([[:File:M97Ch1.2FG8.jpg|Figure 8]]). Oil saturations range from 63 to 80% of pore volume, with porosities of approximately 5 to 6% in this interval (Tanck, 1997). A deeper zone at 2075.7 to 2080.3 m (6810–6825 ft) has similarly high oil saturations, but much lower porosities in the 3 to 4% range (Tanck, 1997).

The shale intervals are more organic rich and have higher oil contents indicative of source rocks that have generated hydrocarbons. In general, however, the higher the TOC, the lower is the oil crossover. Porosities are also lower in the shale, typically in the range of 2 to 3% (Tanck, 1997).

The shale intervals are more organic rich and have higher oil contents indicative of source rocks that have generated hydrocarbons. In general, however, the higher the TOC, the lower is the oil crossover. Porosities are also lower in the shale, typically in the range of 2 to 3% (Tanck, 1997).