Sedimentological and Diagenetic Controls on the Reservoir Quality of the Snadd Formation (Triassic), Barents Sea, NO

Neil McDougall

A regional study of the Ladinian-Carnian Triassic Snadd Formation in the Norwegian Barents Sea integrates conventional logs, seismic, >400m of detailed core descriptions from 15 wells, petrographic and petrophysical data to define depositional environments, paleogeographies, and sequence stratigraphic framework, allowing identification of the main factors controlling reservoir quality. The Snadd Formation was divided into two informal members: Lower and Upper Snadd, comprising 14 facies associations deposited in a series of environments ranging from braided to locally meandering fluvial channels and sheetfloods to lower shoreface and inner shelf. High resolution correlations recognize two major sequences and up to 7 higher order sequences, in which shelf margin/low stand (LST/SMST), transgressive (TST) and highstand system tracts (HST) were identified. Paleogeographic maps based on core descriptions and interpretations of uncored sections from 23 wells were also prepared for Lower and Upper Snadd. For the Lower Snadd, thickness increases from east to west, but there is some evidence for a platform break and clinoforms in the NW of the study area; the unit is characterised by inner shelf to lower shoreface deposits with only a localized coastal braidplain in the SE. The Upper Snadd also thickens from east to west with the exception of the paleo-Loppa High, and is characterized by flat parallel to sub-parallel reflectors and a shift from a semi-arid floodplain to offshore shallow marine. Petrographic samples taken from 13 wells show that reservoir sandstones are mostly fine- to medium-grained, feldspathic litharenites and litharenites with abundant acidic volcanic rock and chert fragments (Lower Snadd avgQFR=58-14-28; Upper Snadd avgQFR=44-18-38). Two main diagenetic processes have critically impacted reservoir quality by retarding mechanical compaction and impeding quartz cement precipitation in intergranular pores: 1) early siderite precipitation –either as microgranular rims on grains, or pore filling with micronodular texture-, and 2) precipitation/recrystallization of chlorite as well developed grain coats, which are especially abundant within the VRF- and Chert-enriched sandstones of fluvial or freshwater-influenced marine settings of the Upper Snadd. Early precipitation of siderite cement is notably more common in SMST/LST sandstones. Similarly, chlorite coats are also much better developed in SMST/LST and TST, than in HST sandstones. A direct relationship between chlorite coats/quartz cement inhibition and poroperm properties measured in cores can be traced. Also, analysis of size distribution of optical porosity elements (“porels”) in Lower vs. Upper Snadd obtained by QEMSCAN® help to explain observed differences in permeability between the two subunits. Thus, some of the internal variations in reservoir quality of the Snadd can be explained, and even predicted, by using a detailed facies analysis and sequence stratigraphic framework.

AAPG Search and Discovery Article #90177©3P Arctic, Polar Petroleum Potential Conference & Exhibition, Stavanger, Norway, October 15-18, 2013