Petroleum geochemistry of the so-called “dry-wells” off mid-Norway; clues to ignored and overlooked petroleum systems

Abstract

Forty-eight (48) core samples from well 35/4-1, 6610/3-1 and well 6610/2-1S were obtained from the North Sea and the Norwegian Sea. These wells are classified by NPD as dry. These core samples have been studied geochemically, and also using microscopy to look for petroleum inclusions. All core samples from these wells have been taken from the anticipated reservoir units and are mainly sandstones and cemented sandstones but also partly sandstone/siltstones. The focus of this study is to try to carry out geochemical analysis of the samples to characterize the gas from inclusions and the core extracts, provided in these so called dry wells off Mid-Norway. An important element of these investigations is also to try to document, using UV-microscopy, if any petroleum inclusions are present. If such are found, this will signify that the “dry wells” did earlier contain petroleum. Furthermore, the existence of core extracts representing migrated petroleum signifies that the “dry wells” may indeed have intersected what in the pre-history was a paleo-oil charge. Destructive analytical methods like gas analysis are used to define the presence of HC gas in inclusions, and GC-FID of core extracts is used to determine if allochthonous bitumen exists in the core material. Chromatograms are produced from destructive analytical methods to determine the presence of light HCs in inclusions in mineral cements, and bitumen in the core samples and on the basis of these chromatograms, different source rock facies and maturity parameters were computed. Using these data, cross plots were generated to evaluate dry/wet gases, maturity and the organic facies of the bitumen. Visual examination of petroleum inclusions in these core samples was also used to define the presence of paleo-gas and oil charges in the samples. Well 35/4-1 from North Sea is rich in light HCs and n-alkanes but the majority of the samples are biodegraded and well 6610/3-1 is also rich in light HCs but less amount of n-alkane signatures are recorded on chromatograms. Well 6610/2-1S contains HC gas bearing inclusions and also fluorescent inclusions and there are no n-alkanes signatures recorded. This is due to massive core contamination by circulating drilling fluids. The fact that this well contains oil type inclusions proves that oil was present as a mobile phase. Thus, the core samples studied from well 6610/2-1S has under microscope shown oil signature in UV light which has indicated that there would have been detectable n-alkane on chromatograms produced by GC-FID, but unfortunately was the core extracts heavily contaminated by polyethylene glycols, rendering further analytical work useless. Although light HCs are present in inclusions in all three wells, bitumen representing truly migrated petroleum could only be extracted from well 35/4-1 and 6610/3-1. The core samples from the well 35/4-1 are richer in HCs than in well 6610/3-1. The core samples from the well 35/4-1 show higher maturity and less biodegradation, while the well 6610/3-1 shows lower maturity and more biodegradation than the well 35/4-1. The source rock facies from the well 35/4-1 is clearly a type II source rock whilst well 6610/3-1 also show a type II source rock facies, albeit with some more terrestrial input. A likely source rock facies of well 35/4-1 is distal Draupne, while a more proximal Spekk Formation is the likely source for the petroleum found in well 6610/3-1. It is concluded that both well 35/4-1 and 6610/3-1 intersects paleo-reservoir units of either a migration an avenue or a reservoir and that thermogenic petroleum at one time migrated in this system. This has implications to further exploration in these regions.