Integrating Wireline Interpretation with Rock Properties to Better Determine Controls on the Occurrence of Reservoir Sand in Pohokura Field, Taranaki Basin, New Zealand

Abstract

This study of reservoir sands in two wells in the Eocene Mangahewa Formation Pohokura Field, New Zealand is designed to show how integrating field-scale information from FMI interpretation with existing core and rock property data helps in gaining a better understanding the reservoirs sands. Specifically, it aims to quantify controls on the behav- ior of the reservoir’s porosity and permeability. In the Pohokura-1 well, lamination and cross lamination dips are pre-dominantly SE and NW and subparallel to Pohokura anticlinal structure. In contrast, dips of lamination in the Pohokura South-1 well show more polymodal trends. These differences in lamination trend indicate differences in sand body orientation, which under petrographic study translates to the sand bodies in each well experiencing different depositional or tectonic histories. There are different sources of sediment supply and hence rock type due to different paleocurrent directions (as indicated by differences in cross lamination directions). Depositional environ-ments, (not diagenesis) play a fundamental role in determining reservoir quality as they control the character of the original pore network in the newly deposited sediments. Massive Sandstone lithofacies (as defined in FMI) is seen to form the best quality reser- voir where muddy lamination and bioturbation are seen to be detrimental to the reservoir quality. Overall, each of the documented diagenetic factors shows a damaging effect on the reservoir quality, but grain size and sorting dominate. Subsequent compaction and cementation are more critical post depositional effects compared to authigenic clay content and grain dissolution. There is a positive relationship between porosity-permeability and total quartz cement (levels <6% quartz overgrowth) as it sets up a stable grain framework that resists compaction. Beyond 6%, higher amounts of quartz overgrowth cement are detrimental as the cement is now present in sufficient pore volumes to occlude pore throats and lower permeability. In conclusion, differences in sand body geometry and probably different sources of sediment supply, together with textural, compositional and diagenetic variations between wells result in differences in reservoir quality. Pohokura-1 has a relatively better reservoir quality than Pohokura South-1 as the reservoir sand is coarser grained, and more quartzose dominant. In contrast, reservoir sands in Pohokura South-1 are finer-grained and more diagenetically active due to inherently higher feldspar and lithic contents.