Petrography reveals controls on thermal history using evidence from mineralogy and microstructure in addition to accurate maturity data and recognition of paleo-maturity signatures. The scope of VIRF analysis includes all of these areas.
Igneous and hydrothermal activity leave a distinctive suite of petrographic markers that we see in petroleum basins all over the world. Convective heat flow is a commercially important but frequently unrecognised control on thermal history, and effective thermal history modelling needs to include options for convective heat flow.
Using part of the Williston Basin as an example, this schematic shows that igneous volatiles, hydrothermal fluids, and hydrocarbons travel convectively through fault zones and rock units that have high permeability at the time of intrusion. “Hot spots” can be “sweet spots” for hydrocarbon production (Edman et al. 2015).
Fig.1 from Janell Edman, Eve Sprunt, Jane Newman, Michal Ruder, and Jim Ellis (2015). ”More efficient and cost-effective ways of evaluating and high grading unconventional plays.” Interpretation, 3(3), SU33-SU46.
Coke and pyrolytic carbon form at extreme temperatures that can only be achieved as a result of magmatic activity. The example below shows pyrolytic carbon in a North Sea exploration well; (a) thin carbon layers have accreted successively as a sphere that was repeatedly suspended in a superheated vapour stream; (b) a compound example with deep fractures containing petroleum coke.
Fig.2 from Newman, J., Newman, R., Gize, A and J. Edman (2017) A petrography-based model of igneous and hydrothermal activity in diverse petroleum basins. AAPG Geoscience Technology Workshop, Influence of Volcanism and Associated Magmatic Processes on Petroleum Systems. Oamaru, New Zealand, March 2017. (Incident light, partially crossed polars, oil immersion).
Sills, dykes and extrusive lavas are sometimes evident on seismic profiles and wireline logs, but this is not always the case. Some intrusions are too thin to be detected, and others have been altered to clays or have anomalous compositions that are invisible to remote sensing. In these cases petrography reveals an intrusive history that has not previously been suspected.
Microfacies VS (see image below) is a cryptic melt rock that has been logged incorrectly as shale in petroleum wells all over the world. Petrography reveals a glassy framework outlining the angular shapes of phenocrysts (p) and the rounded shapes of gas vesicles (v), as shown by this example at 2170m in the Waka Nui-1 exploration well (New Zealand).
Fig.3 from Newman, J., Newman, R., Gize, A and J. Edman (2017) A petrography-based model of igneous and hydrothermal activity in diverse petroleum basins. AAPG Geoscience Technology Workshop, Influence of Volcanism and Associated Magmatic Processes on Petroleum Systems. Oamaru, New Zealand, March 2017. (Incident polarised light, oil immersion).
Igneous and hydrothermal influences during basin history typically result in complex maturity profiles, but this complexity is frequently not recognised due to a reliance on poor quality VR datasets. The sensitivity of VIRF maturity data reveals that although convective heating can cause rapid increases in maturity with depth it can also result in stratigraphic intervals which exhibit no increase in maturity over substantial thicknesses of strata. These intervals of uniform maturity typically had high primary porosity and permeability and allowed unrestricted circulation of hydrothermal fluids within a 'paleo-aquifer' zone. Subsequent occlusion of this porosity by silica and carbonate leaves distinctive microfabrics that can be recognised petrographically. The sample below is from a paleo-aquifer zone in a North American Cretaceous succession.
Incident light, oil immersion.
The analytical resolution provided by VIRF analysis has also allowed us to recognise fossil maturity profiles. The chemistry of vitrinite and other organic constituents is sensitive to chemically aggressive igneous volatiles that are released as magma rises up through the succession. These volatiles travel preferentially along paths of least resistance, including ‘paleo-aquifer’ units with high primary permeability and rocks that are pervasively fractured.
Chemical damage by igneous volatiles prevents further organic maturation. The maturity of affected vitrinite is frozen or “write-protected” and provides a snap-shot of thermal history and source rock maturity before intrusion occurred. In the example below, VIRF analysis allows the fossil maturity population in this composite sample to be distinguished.
Fig.5 From Newman, J., J. D. Edman, J. A. LeFever, J. Howe, 2013. Parshall Field: Inferences from new data regarding Bakken hydrocarbon generation and migration (extended abs.): 2013 Unconventional Resources Technology Conference, #1578764.
References:
Newman, J., J. D. Edman, J. A. LeFever, J. Howe, 2013. Parshall Field: Inferences from new data regarding Bakken hydrocarbon generation and migration (extended abs.): 2013 Unconventional Resources Technology Conference, #1578764.
Janell Edman, Eve Sprunt, Jane Newman, Michal Ruder, and Jim Ellis (2015). ”More efficient and cost-effective ways of evaluating and high grading unconventional plays.” Interpretation, 3(3), SU33-SU46.
Newman, J., Newman, R., Gize, A and J. Edman (2017) A petrography-based model of igneous and hydrothermal activity in diverse petroleum basins. AAPG Geoscience Technology Workshop, Influence of Volcanism and Associated Magmatic Processes on Petroleum Systems. Oamaru, New Zealand, March 2017.