Geological and Geophysical Investigation of Palaeokarst and its Association with Faults and Fractures on the Loppa High,SW Barents Sea

Abstract

The reservoir quality of the prospective Palaeozoic carbonates on the Loppa High, Norwegian Barents Sea, is believed to be controlled by the original heterogeneous depositional facies which is linked to active faulting and which has secondarily been overprinted by palaeokarst. Studies of carbonate buildups on the Finnmark and Bjarmeland platforms, and on the Loppa High have shown that they are not isolated but that carbonate ridges are linked into interconnected buildup mosaics. Studies from these areas have also shown that the buildups were controlled by faults and seafloor morphology at the time of their growth. The carbonate buildups formed during a period of approximately 35-40 Ma starting in the Moscovian and ending in the Artinskian. The Barents Sea area was part of the northern Pangaea shelf during this period which drifted from a subtropical position at approximately 30˚N to a temperate position at around 45˚N. The carbonates were influenced by high frequency and high amplitude, glacioeustatic sea-level fluctuations during deposition.

An important class of carbonate petroleum reservoirs are karst-related palaeocave systems. It is essential to understand the formation and spatially extent of such systems for exploration and development purposes. The Loppa High was subaerially exposed for approximately 25Ma, which led to widespread karstification. Karst structures on the Loppa High have been reported to be controlled by the positions of faults and fractures in the underlying sediments. In order to further explore this relationship, Shclumberger‟s Ant Tracking algorithm, which is very efficient for finding and extracting faults and fractures, has been applied to two 3D seismic surveys from the Loppa High, i.e. survey SG9810 and site survey NH0372. Kovacova (2010) tested this algorithm on the same surveys in order to find a possible relationship between the position of carbonate buildups and faults/fractures, which were proven to be the case. The precise location of faults and fractures could not be determined by the application of this algorithm alone, but a structural trend was identified on the results from site survey NH0372. Fault controlled karst is very realistic from a geological point of view and the karstified carbonates on the Loppa High is therefore also strongly believed to be controlled by faults and fractures based on results from the Ant Tracking workflows and other attributes tested on the seismic data.