--> Abstract: Improved Determination of Source Quality and Kerogen Type by Combining Rock-Eval and Pyrolysis-Gas Chromatography Results, by H. Dembicki, Jr.; #90987 (1993).

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DEMBICKI, JR., HARRY, Marathon Oil Company, Littleton, CO

ABSTRACT: Improved Determination of Source Quality and Kerogen Type by Combining Rock-Eval and Pyrolysis-Gas Chromatography Results

Rock Eval pyrolysis is widely used as a means of determining the type of organic matter (kerogen) present in a sediment and the type of petroleum it is likely to generate (source quality). However, Rock-Eval data by itself is not always adequate for making these determinations. When mixtures of kerogen are present, the kerogen type and source quality of a sediment may not be accurately indicated. For example, mixtures of Type I and III kerogens can be interpreted as Type II kerogen and, when Type IV (inert) kerogen is present, source rocks that could generate oil may appear to be gas-prone. In addition, the mineral matrix can also influence interpretation of Rock Eval data. Carbonate minerals can increase the oxygen index, while mixed layer clays can lower the hydrogen index. These re ults indicate that Rock Eval data needs to be supplemented by other organic geochemical data to obtain accurate appraisals of source rocks.

One solution to this problem is to use pyrolysis-gas chromatography in addition to Rock Eval analysis. The advantage of pyrolysis-gas chromatography is that it provides the composition of the pyrolysis products of source rock samples. With this compositional information, kerogen type and/or source quality of sediment are directly and more easily determined. When Rock Eval data is used in conjunction with pyrolysis-gas chromatography data, kerogen mixtures in source rocks can be recognized and interpreted, dilution of generative kerogen with inert Type IV kerogen can be detected, and some of the mineral matrix effects can be compensated for.

Examples will be shown where source rocks with oil generative potential were thought to be gas-prone using Rock Eval data alone. Recognition of overlooked oil-prone sediments, such as these, can lower the risk of exploration plays and increase the potential for prospects to be drilled.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.