Seal Character and Variability in Tertiary Submarine Fan Petroleum Systems
Analyses of petroleum systems developed in Tertiary-aged submarine fan depositional settings reveal that shale properties and seal character vary systematically and exhibit moderate to strong correlations with sedimentological parameters and sequence stratigraphic position. In particular, enhanced sealing characteristics are associated with well-developed laminar fabrics, organic matter enrichment, and early marine carbonate cementation, whereas fabric disruption (bioturbation and slumping) generally degrades seal properties. Additionally, increased percentages of silt-sized siliciclastic and bioclastic grains allow preservation of relatively large-diameter pore throats, thereby reducing sealing capacities. In contrast, silt-poor (<10%) shales can have excellent to exceptional sealing properties. Because of variations in fabric and texture, different shales exhibit distinctive compaction trends. Hence, applying a default compaction trend can result in erroneous interpretations of burial history and erroneous interpretations of timing for hydrocarbon migration events from basin models. Stacking patterns within submarine fan stratal packages commonly induce multiple scales of cyclicity and superimposition of units having broadly varying sealing characteristics. Laterally extensive top seals for these petroleum systems record deepening and subsequent transgressive events; these “high-quality” transgressive shales separate lowstand intervals containing lower quality seals (baffles) and sandstone reservoir packages. Generalized models for channelized submarine fans allow facies recognition from seismic which, in turn, permits mapping of shale/seal facies from amplitude variations evident within horizon slices. Seal disruption by sand injection and associated fractures and faults is a significant risk in these petroleum systems allowing hydrocarbon leakage between stratigraphic compartments, ultimately resulting in potentially thick waste zones.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009