Petrophysical implications of source rock microfracturing

作者：Kalani Mohsen^*; Jahren Jens; Mondol Nazmul Hague; Faleide Jan Inge

来源：International Journal of Coal Geology, 2015, 143: 43-67.

DOI：10.1016/j.coal.2015.03.009

• Hydrocarbon generation induced microfracturing
• Petrophysical properties
• Formation resistivity
• P-wave velocity (Vp)
• Cross-plot relationship
• Clays

Hydrocarbon generation induced microfracturing in source rocks is important affecting both primary migration of hydrocarbons, and unconventional shale plays. In this study petrographic observations, Rock-Eval and total organic carbon content (TOC) data, and log-derived physical properties were analyzed in order to evaluate eventual petrophysical implications of microfracturing induced by hydrocarbon generation.
Petrographically, evidences of microfracturing both normal and parallel to bedding have been found. Horizontal microfractures typically cross cut each other at an angle between 30 and 90 degrees. Horizontal microfractures are readily imaged compared to vertical microfractures. Vertical microfractures were clearly observed only in rare cases in samples with a high degree of microfracturing. In the case of moderate microfracturing, the vertical component of microfractures may be too small to be detectable using conventional microscopy techniques. Longer vertical components of the microfractures may be facilitated by higher TOC and particularly dense laminated organic matter fabric, as well as higher thermal maturity levels. Clay minerals, particularly smectites are also playing a role in reducing permeability and thereby increasing the local pressure generated by hydrocarbon generation eventually resulting in microfracturing.
Log-derived petrophysical properties particularly P-wave velocity (Vp) and formation resistivity can give information on microfracturing potential even though the techniques cant resolve the microfractures directly. Vp has been postulated to be sensitive to organic matter quantity and fabric, whereas formation resistivity is largely governed by fluid content and clay minerals. A reverse trend of increasing resistivity with decreasing Vp was found and is linked to higher hydrocarbon saturation and TOC and/or dense laminated fabric of organic matter favoring microfractures. The implications of the log-derived petrophysical results are in agreement with the direct petrographic observations. Formation resistivities in the source rocks, however, are lower than those observed in non-clay dominated hydrocarbon saturated intervals. This indicate that low permeable organic rich source rocks with a high clay content, particularly smectites, may facilitate formation of the hydrocarbon filled vertical microfractures even at relatively low thermal maturities.

• 出版日期2015-4-1