Silica diagenesis, polygonal faulting, and shallow gas: implications for fluid migration, storage, and shallow hazards

Abstract

By integrating 3D seismic data, petrophysical well logs and well cuttings samples, three (3) broad areas related to hydrocarbon plumbing systems and subsurface fluid flow in the eastern Central North Sea are investigated in this research, silica diagenesis, polygonal faulting, and shallow gas accumulations. Silica diagenesis which involves a two-step process of the conversion of amorphous biogenic silica (Opal-A) to crystals of cristobalite or tridymite (Opal-CT) and subsequently to crystals of quartz has been identified in sedimentary basins around the world. This process has the potential of significantly affecting the physical, mechanical, and fluid flow properties of the host rock and are important to the development of sedimentary basins. This study identifies for the first time, the presence of an Opal-A/CT reaction front in the Cenozoic mudstones of the eastern CNS by the use of a range of techniques including X-ray diffraction data (XRD) and the Quantitative Evaluation of Minerals by Scanning Electron Microscope (QEMSCAN) and conventional 3D seismic and well data interpretation techniques. Further, we analysed the impact the presence of salt diapir in the study area and conclude that it helped in elevating temperatures locally which led to the fossilization of the silica diagenetic reaction front. Furthermore, we investigate the relationship between silica diagenesis and polygonal fault systems hosted within the same interval. Recently, there has been a growing link between silica diagenesis in biosiliceous sediments and the evolution of polygonal faults. We investigated this link using conventional 3D seismic and well data through a spatial, temporal, and kinematic analysis of the polygonal fault systems within the sediments. We proposed a model for the nucleation, growth, and propagation of the polygonal fault system as a contemporaneous process happening alongside silica diagenesis and report that most of the faults are in-active at present, except for a few breaching the mid-Miocene unconformity surface and this is contemporaneous also, with the fossilization of the Opal-A/CT reaction front. Lastly, we investigated twenty-six shallow gas accumulations appearing as ‘bright spots’ on seismic data as seismic anomalies within the Cenozoic succession using conventional 3D seismic interpretation methods and geochemical methods for the evaluation of organic matter richness and thermal maturity from well cuttings sample within the shallow section. We classified the shallow gas accumulations based on their on their direct hydrocarbon indicator (DHI) characteristics, spatial and temporal distribution, and their relationship with other focused fluid flow related features into two types, Type I & Type II. The Type I shallow gas anomalies are often found associated with the Zechstein salt diapirs while the Type II anomalies are found in discreet pockets within Delta Front sediments. Geochemical analysis from well cuttings samples indicates an average total organic carbon (TOC) content of 5% and with a very good generative potential, suggesting the possibility of the charging of shallow reservoirs hosting the shallow gas accumulations by biogenic gas in combination with deep thermogenic sources. We present a model for the shallow gas accumulations within the study area which may assist in mitigating risks associated with shallow gas accumulations whether they are considered as a shallow geohazard for the drilling of deeper targets or a potential new play where they are located near existing infrastructure. This study has implications for fluid migration, subsurface storage of carbon dioxide and nuclear waste and the assessment of shallow geohazards.

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Mads Huuse (Supervisor)