Predictive Modeling and Timing of Porosity-Preserving Events in Maastrichtian-Danian Chalk Reservoirs of the North Sea

Peter A. Scholle¹, Henrik Tirsgaard², and Lene Clausen^2
1 New Mexico Bureau of Geology, NM Tech, Socorro, NM
² Maersk Olie og Gas AS, DK-1263 Copenhagen K, Denmark

Two major diagenetic factors are known to preserve high porosities in North Sea chalks: inhibition of chemical diagenesis resulting from the presence of hydrocarbons, and the physical and chemical effects of anomalous pore fluid pressures (overpressuring). Predicting the timing and regional trends of these factors prior to drilling is critically important in chalk exploration.

A technique was developed that combines measured chalk porosities, quantitative burial history information, and predictive porosity-depth models to calculate minimum ages at which overpressure onset (or other inhibition factors) had to have occurred in order to preserve present-day porosities. The resulting value is termed the minimum inhibition age (MIA). Using data exclusively from non-reworked chalks containing no hydrocarbons one can calculate regional chalk MIAs that primarily reflect the timing of overpressure onset. MIAs can also be calculated for overlying Eocene shales — they too reflect times of overpressure onset rather than hydrocarbon entry.

Extensive data from the Danish sector show significant regional variations in MIAs: from an average of 13-16 myBP in the northern and central Danish North Sea, to less than 8-10 myBP in the southwest. These MIAs agree well with times of rapid sediment loading and inferred hydrocarbon migration in their respective areas. Furthermore, detailed analysis of differences between shale and chalk MIAs in selected wells provides information on local variations in timing of hydrocarbon entry.

Because porosity and burial history information can also be determined from seismic interpretation, MIA calculations can be made in undrilled areas. Such pre-drilling recognition of regions with early onset of overpressuring or early hydrocarbon entry may lead to upgrading of prospects in such areas.