Changes

Although an oil crossover value of less than 100 mg HC/g TOC does not rule out the possibility of having producible oil, it does represent substantially higher risk based strictly on geochemical results. It may be that samples have been dried or more volatile liquids have evaporated, particularly in conventional reservoir lithofacies.

Although an oil crossover value of less than 100 mg HC/g TOC does not rule out the possibility of having producible oil, it does represent substantially higher risk based strictly on geochemical results. It may be that samples have been dried or more volatile liquids have evaporated, particularly in conventional reservoir lithofacies.

Finally, it is not only important to locate oil reservoirs, it is important also to assess the quality of the oil contained in the reservoir. Such techniques have been described<ref>Jarvie, D. M., A. Morelos, and Z. Han, 2001a, [http://www.wwgeochem.com/resources/Jarvie+2001+Williston+Basin+Petroleum+Systems+paper.pdf  Detection of pay zones and pay quality, Gulf of Mexico]: Gulf Coast Association of Geological Societies Transactions 51st Annual Convention, Shreveport, Louisiana, October 17–19, v. LI, p. 151–160.</ref> and are an essential part of assessing the economic value of a reservoir. Basic tests include determination of sulfur content, API gravity, viscosity, and yield of polar resin and asphaltene relative to nonpolar saturate and aromatic hydrocarbons. A quick screening approach is to use gas chromatography to predict oil quality based on the fingerprint derived from the rock extract; this is the same tool used on produced oil samples or recovered from reservoir tests.

Finally, it is not only important to locate oil reservoirs, it is important also to assess the quality of the oil contained in the reservoir. Such techniques have been described<ref>Jarvie, D. M., A. Morelos, and Z. Han, 2001a, [http://www.wwgeochem.com/resources/Jarvie+2001+Williston+Basin+Petroleum+Systems+paper.pdf  Detection of pay zones and pay quality, Gulf of Mexico]: Gulf Coast Association of Geological Societies Transactions 51st Annual Convention, Shreveport, Louisiana, October 17–19, v. LI, p. 151–160.</ref> and are an essential part of assessing the economic value of a reservoir. Basic tests include determination of sulfur content, API gravity, viscosity, and yield of polar resin and asphaltene relative to nonpolar saturate and aromatic hydrocarbons. A quick screening approach is to use [[gas chromatography]] to predict oil quality based on the fingerprint derived from the rock extract; this is the same tool used on produced oil samples or recovered from reservoir tests.

==Oil crossover effect examples==

==Oil crossover effect examples==

The TOC values are high in the upper Bakken Shale, averaging 14.3%, with values ranging between 5.36 and 21.40%, and they are just slightly higher in the lower Bakken Shale at 15.17%, with a range from 8.87 to 24.7%. Carbonate contents in the upper and lower Bakken Shale average 10 and 6%, respectively. The carbonate-rich Scallion above the upper Bakken Shale and Middle Member are readily recognizable, with their high carbonate and low TOC contents. Similar results are found in the Three Forks Formation underlying the lower Bakken Shale. The carbonate content in the Middle Member of the Bakken Formation is primarily dolomite and averages approximately 38%, with a range between 21 and 70%.

The TOC values are high in the upper Bakken Shale, averaging 14.3%, with values ranging between 5.36 and 21.40%, and they are just slightly higher in the lower Bakken Shale at 15.17%, with a range from 8.87 to 24.7%. Carbonate contents in the upper and lower Bakken Shale average 10 and 6%, respectively. The carbonate-rich Scallion above the upper Bakken Shale and Middle Member are readily recognizable, with their high carbonate and low TOC contents. Similar results are found in the Three Forks Formation underlying the lower Bakken Shale. The carbonate content in the Middle Member of the Bakken Formation is primarily dolomite and averages approximately 38%, with a range between 21 and 70%.

Continuous oil crossover is present in both the Scallion and Middle Member, with the Middle Member being the principal reservoir that is now drilled horizontally. Although a particular zone in the Middle Member, for example, the B zone,<ref>Oil & Gas Journal, 2010c, [http://www.pennenergy.com/index/petroleum/display/6670774195/articles/oil-gas-financial-journal/volume-6/Issue_7/Features/Whiting_Petroleum_s__sweet_spot__is_most_prolific_part_of_the_Bakken.html Whiting Petroleum's sweet spot is most prolific part of the Bakken].</ref> is preferred by operators, the entire Middle Member is highly oil saturated. Absolute oil contents average about 0.00747 m3/m3 (58 bbl/ac-ft) in the Middle Member, whereas the Scallion has a much lower average of 0.00141 m3/m3 (11 bbl/ac-ft). Both of these values are based on absolute oil (S1) yields, and based on a comparison of rock extracts with produced oil, a substantial loss of hydrocarbons is evident in the rock extracts, with minimal C15- measured by gas chromatography.<ref name=Jetal2011 /> The upper Bakken Shale has a fingerprint nearly identical to the oil, whereas the Middle Member fingerprint looks like a topped (evaporated) oil.<ref name=Jetal2011 /> This illustrates an important difference between the organic-rich shales and the carbonates, as all samples were core chips taken at the same time. The organic-rich shale retains even light hydrocarbons from C5 to C10, whereas the organic-lean carbonate appears as a C15+ extract fingerprint with loss of light ends. The difference is not primarily caused by permeability differences, but retention (sorption) by the organic-rich mudstones of the Bakken shales. Although the Bakken Shale-oil yields (S1) are much higher than the Scallion and Middle Member free oil contents due to much evaporative loss, only a part of the oil in the shale would be producible, i.e., only excess oil exceeding the adsorption index (AI).

Continuous oil crossover is present in both the Scallion and Middle Member, with the Middle Member being the principal reservoir that is now drilled horizontally. Although a particular zone in the Middle Member, for example, the B zone,<ref>Oil & Gas Journal, 2010c, [http://www.pennenergy.com/index/petroleum/display/6670774195/articles/oil-gas-financial-journal/volume-6/Issue_7/Features/Whiting_Petroleum_s__sweet_spot__is_most_prolific_part_of_the_Bakken.html Whiting Petroleum's sweet spot is most prolific part of the Bakken].</ref> is preferred by operators, the entire Middle Member is highly oil saturated. Absolute oil contents average about 0.00747 m3/m3 (58 bbl/ac-ft) in the Middle Member, whereas the Scallion has a much lower average of 0.00141 m3/m3 (11 bbl/ac-ft). Both of these values are based on absolute oil (S1) yields, and based on a comparison of rock extracts with produced oil, a substantial loss of hydrocarbons is evident in the rock extracts, with minimal C15- measured by gas [[chromatography]].<ref name=Jetal2011 /> The upper Bakken Shale has a fingerprint nearly identical to the oil, whereas the Middle Member fingerprint looks like a topped (evaporated) oil.<ref name=Jetal2011 /> This illustrates an important difference between the organic-rich shales and the carbonates, as all samples were core chips taken at the same time. The organic-rich shale retains even light hydrocarbons from C5 to C10, whereas the organic-lean carbonate appears as a C15+ extract fingerprint with loss of light ends. The difference is not primarily caused by permeability differences, but retention (sorption) by the organic-rich mudstones of the Bakken shales. Although the Bakken Shale-oil yields (S1) are much higher than the Scallion and Middle Member free oil contents due to much evaporative loss, only a part of the oil in the shale would be producible, i.e., only excess oil exceeding the adsorption index (AI).

In addition, the high remaining generation potentials (Rock-Eval S2) in the Scallion and Middle Member are not kerogen content, but instead oil that has carried over into the pyrolysis (S2) yield. This is also noted by the lower equivalent Ro values in the Scallion and Middle Member data. Addition of this carryover oil to the free oil gives the total oil.

In addition, the high remaining generation potentials (Rock-Eval S2) in the Scallion and Middle Member are not kerogen content, but instead oil that has carried over into the pyrolysis (S2) yield. This is also noted by the lower equivalent Ro values in the Scallion and Middle Member data. Addition of this carryover oil to the free oil gives the total oil.