Introduction
Conventional and tight sandstone reservoirs are still prominent targets for oil and gas exploration. Although well known, any kind of improved reservoir quality prediction may improve the return of invest. Our approach is a better understanding of reservoir quality in a 3D carbonate- and siliciclastic rock body based on thorough reservoir characterization using novel approaches (Becker et al., 2018a; Becker et al., 2017a; Busch et al., 2019; Busch et al., 2018; Wüstefeld et al., 2018; Wüstefeld et al., 2017) With a focus on siliciclastic rocks, we present an overview of our approaches. We show that micrometer-thin clay coatings have a tremendous effect on reservoir quality, that quartz overgrowth can be used to calibrate subsidence curves and that faults may act as heat conduits affecting the maturation of fields on the reservoir scale.
Geological Setting
Our target areas are the conventional and tight, gas-producing siliciclastic fields of Northern Germany, which can be found in the Rotliegend eolian red bed formations (with reservoir qualities in the range of 0.01 to 1000 mD) and the Upper Carboniferous sandstone units (with reservoir qualities in the range of 0.01 to 100 mD). ... mehrBoth reservoirs are subsalt, and thus, seismic underground mapping is challenging due to poor resolution. We compare subsurface reservoir rocks with reservoir analogous rocks exposed at the surface to get a better understanding of 3D structural and sedimentary heterogeneities, well knowing that neither rock composition nor structure fully match subsurface conditions.
Results
Our results on red bed reservoirs show, that an increased amount of illite coating affects reservoir quality. State-of-the-art TouchStone reservoir quality modeling (Busch et al., 2018, Lander et al. 2008) outlines that quartz cementation and pore destruction is inhibited by pore lining clay minerals (Busch et al., 2018). Using detrital and authigenic mineral input data from thin section analyses, together with petrophysical porosity- and permeability data, reservoir quality models can be successfully calibrated to predict porosity- and permeability evolution during basin history (Becker et al., 2018b; Busch et al., 2019; Busch et al., 2018). Natural reservoir data on compaction, cementation and two-phase flow are now also implemented into numerical models (Prajapati et al., 2018) to better constrain flow from grain- to reservoir scale.
Tight gas reservoir quality relies on fracture density and pore volume. Our approach of terrestrial laser scanning allows to detect and quantify fracture heterogeneities, and generate fracture networks (Becker et al., 2018a; Becker et al., 2017b; Wüstefeld et al., 2018). Improved porosity and permeability conditions can be correlated with larger amounts of reservoir uplift, which is subsequently linked to reduced temperatures and thus reduced quartz precipitation rates (Becker et al., 2018b; Busch et al., 2019).
The role of faults with 100s meters of displacement is shown to be two-fold. Additional fracturing in their damage zones can provide flow paths but they can also act as heat conduits. Our Piesberg tight gas sandstone analog study clearly points to overmature conditions in an overall mature area due to the presence of a large normal fault where high temperature fluids could migrate (Wüstefeld et al., 2017). In contrast, in the Rhine graben area, reservoirs represent mature fields in an overall immature basin.
Outlook
The correlation of a thorough diagenetic study associated with petrophysical porosity and permeability analyses, together with reservoir quality prediction modeling, better unravels the evolution of reservoirs and may result in new prospects. Anisotropic, structural related paleo-heat flow analyses may also guide exploration towards new plays not yet considered and similarly reduce the risk of drilling in overmature reservoirs.
Digital tools are underway to improve two-phase flow in compacting and cementing reservoirs and natural digital fracture network models. Combined with artificial intelligence one may analyze yet unknown data volumes and thus, improve the estimates of production potential.
References
Becker, I., Koehrer, B., Waldvogel, M., Jelinek, W., Hilgers, C., 2018a. Comparing fracture statistics from outcrop and reservoir data using conventional manual and t-LiDAR derived scanlines in Ca2 carbonates from the Southern Permian Basin, Germany. Marine and Petroleum Geology 95, 228-245.
Becker, I., Wüstefeld, P., Koehrer, B., Busch, B., Hilgers, C., 2018b. Developing an exploration strategy for Upper Carboniferous tight gas sandstone reservoirs. 80th EAGE Conference and Exhibition 2018, Copenhagen.
Becker, I., Wüstefeld, P., Koehrer, B., Felder, M., Hilgers, C., 2017a. Porosity and permeability variations in a tight gas sandstone reservoir analogue, Westphalian D, Lower Saxony Basin, NW Germany: Influence of depositional setting and diagenesis. Journal of Petroleum Geology 40, 363-389.
Becker, I., Wüstefeld, P., Koehrer, B., Jelinek, W., Hilgers, C., 2017b. Digitally derived fracture network relationships in fractured Zechstein carbonate reservoirs from surface and well data. 79th EAGE Conference and Exhibition 2017.
Busch, B., Becker, I., Koehrer, B., Adelmann, D., Hilgers, C., 2019. Porosity evolution of two Upper Carboniferous tight-gas-fluvial sandstone reservoirs: Impact of fractures and total cement volumes on reservoir quality. Marine and Petroleum Geology 100, 376-390.
Busch, B., Hilgers, C., Lander, R.H., Bonnell, L.M., Adelmann, D., 2018. Reservoir quality and burial model evaluation by kinetic quartz and illite cementation modeling: Case study of Rotliegendes, north Germany. AAPG Bulletin 102, 293-307.
Lander, RH., Larese, RE, Bonnell LM 2008. Toward more accurate quartz cement model: The importance of euhedral versus noneuhedral growth. AAPG Bulletin 92, 1537-1563.
Prajapati, N., Selzer, M., Nestler, B., Busch, B., Hilgers, C., 2018. Modeling fracture cementation processes in calcite limestone: A phase-field study. Geothermal Energy 6, 10.1186/s40517-018-0093-4.
Wüstefeld, P., de Medeiros, M., Koehrer, B., Sibbing, D., Kobbelt, L., Hilgers, C., 2018. Evaluation of a workflow to derive terrestrial light detection and ranging fracture statistics of a tight gas sandstone reservoir analog. AAPG Bulletin 102, 2355-2387.
Wüstefeld, P., Hilse, U., Lüders, V., Wemmer, K., Koehrer, B., Hilgers, C., 2017. Kilometer-scale fault-related thermal anomalies in tight gas sandstones. Marine and Petroleum Geology 86, 288-303.
; Becker, Ivy; Busch, Benjamin