Geochemical Screening of Unconventional Resources of Lower Paleozoic and Older Rocks
Fang Lin and Jozina Dirkzwager
Chevron Energy Technology Company, Houston, TX, USA
Unconventional resources are largely associated with organic-rich shale, which was or is petroleum source rock by conventional definition. Multiple screening methods have been reported for unconventional resource evaluation in the past few years, with most of them focusing on organic richness, thermal maturity and rock failure upon hydraulic fracturing. When applying the methods in specific play evaluation, particularly for lower Paleozoic and older rocks, many issues become prominent. For example, the issue of restoring the initial total organic carbon content (TOC) for mature and over-mature rocks is a difficult one. A popular method used by the industry places strong emphasis on the impact of thermal maturity on TOC. However, measurements from a Silurian shale indicate that thermal maturity may not be the primary control on initial TOC. Instead, original depositional processes may be the predominant factor.
Another issue particularly associated with lower Paleozoic and older rock assessment is the selection of proxies for thermal maturity evaluation. Common maturity parameters such as vitrinite reflectance (VR) are difficult to apply, because true vitrinite particles are rare or absent in rocks of this age. Most of the reported VR for old rocks are derived or calculated based on reflectance of other types of organic matter, such as solid bitumen, marine vitrinitelike maceral, graptolite, conodont etc. Reflectance of these organic matters has different relationships with VR, and the correlation tends to change from place to place depending on where the learning dataset came from. Use of multiple proxies is recommended for cross correlation.
A third issue is related to thermal maturity estimation of source rocks using gas isotope data. As gas isotope rollover becomes a well-known phenomenon in many deep unconventional plays, the traditional method of estimating source rock maturity using gas isotope data becomes suspicious for high maturity gases. Our data analysis suggests that applying the traditional methods on gas data without integrating geologic context may lead to erroneous conclusions. Gas data from unconventional plays should be used with other data to estimate source rock maturities.
A combination of various laboratory measurements may provide a better insight into the spatial extent of factors controlling shale gas generation and in predicting favorable locations for hydraulic fracturing for shale gas production.
AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013