Different aspects of detecting karst with geophysical methods - Tales from the underworld
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The work in this thesis integrate conventional seismic interpretation, gravity survey data, field data from analogues and state-of-the-art numerical modeling in order to broaden and increase the understanding of the distribution of porosity and permeability derived from chemical dissolution of rocks (karst). Karst porosity from small scales to the large are addressed onshore Norway ( Paper 1), the intermediate scale in the offshore setting of the Loppa High, Barents Sea (Paper 2) and the large scale encompassed the entity of both land and offshore areas of Norway (Paper 3). All papers present new workflows combining geophysical methods and geology for karst mapping and prediction. Furthermore, challenges related to distinguishing pseudokarst from karst were addressed in the two offshore papers (Paper 2 and 3), as well as the geophysical imaging problems related to isomorphs (Paper 2). The first case study area (Paper 1) includes onshore Norway’s largest cave room, within the Svarthammarhola cave system (SHC), located in a Caledonian nappe setting in Nordland. This is the first time microgravimetric data is used for mapping of known and detection of hitherto unknown karst cave passages in Norway, and a workflow is presented for interpretation of assumed karst. Cave passages and karst features of the Svarthammarhola cave, form negative density contrasts expressed in gravity field anomalies. Here, microgravimetric methods are shown to be applicable in challenging geological settings with heterogeneous lithologies. This setting is a good onshore analogue which is highly applicable for offshore settings. Challenges due to heterogeneous infill of large cavities and variations in carbonate facies are also addressed. The workflow covers not only the detection of large cave rooms, but also deals with minor karst features (epikarst) in carbonate rocks. A 3D density model was built utilizing 3D forward modelling aiming on matching model with surface gravity measurements. The most important result relates to distinct gravity lows detected in the survey, which are interpreted as formerly unknown and inaccessible cave rooms, some of them of an exceptionally large size. These correspond with known collapse and sediment infill features both at the surface and inside the Svarthammarhola cave. This expands the known cave in an eastward and northward direction. One thought-provoking insight is the cave’s position at the top of the hinge of a large antiform combined with uncommonly high densities in parts of the host-rocks. These observations, together with the cave’s outstanding size, lead to different interpretations regarding the speleogenesis of the Svarthammarhola cave. These interpretations include both meteoric and hypogene karst likely related to hypogenic fluids from deep-seated hotter aqueous flow systems, generally related to tectonic or volcanic systems. Paper 2 deals with different aspects of detecting submerged karst porosity in carbonate rocks using seismic imaging. Several challenges are addressed related to the identification of karst elements and karst landforms. This includes the size distribution of the cavities, the resolution capability of seismic data, the geological history leading to different types of karst infill, and the differentiation from other features which give similar seismic expressions (isomorphs). An important observation here is that identifying karst-like morphology on seismic reflection data alone is not sufficient to prove karstification. The seismic expression needs to be integrated with geological interpretation, and alternative isomorph interpretations need to be assessed. In this paper, the nature of karst features with their seismic expression is reviewed, and potential isomorphs are assembled. Two case studies from the Loppa High of the Barents Sea are presented, where two regions were identified: one with clear karst expressions on seismic data supported by secondary arguments, and one with more equivocal interpretations where alternative models may be valid. The implications of these alternative karst models for hydrocarbon exploration studies are shown to be significant, especially if expected karst-related high porosity zones in fact are absent. Paper 3 unravels karst potential by providing a summary of where to find karst features buried on the NCS and where there is karst potential. Karst (paleokarst) features are occasionally reported in exploration wells throughout the NCS. Still, the contribution of karst porosity to the overall porosity of potential reservoir rocks is highly variable. Meteoric fluids stand out as the primary agents for karstification in this setting, but also hypogene fluids may be expelled upwards during phases of tectonic activity and presumably along deep-seated fault zones. The most promising karst play on the NCS are likely to be found in Late-Paleozoic limestones. Karst in chalk may have a potential upside in the eastern North Sea. Paleokarst may also be found in Caledonian basement terrane, as proven from one well in the Norwegian Sea. The paper also briefly discusses other tertiary porosity elements, such as silicate karst and other pseudokarst features, which might represent a potential upside in reservoir porosity.
Består avPaper 1: Solbakk, Terje; Fichler, Christine; Wheeler, Walter H; Lauritzen, Stein-Erik; Ringrose, Philip. Detecting multiscale karst features including hidden caves using microgravimetry in a Caledonian nappe setting: Mefjell massif, Norway.. Norwegian Journal of Geology 2018 ;Volum 98.(3) s. 359-378
Paper 2: Solbakk, Terje; Svånå, Tore Amund; Fichler, Christine; Ringrose, Philip. Increasing reliability of seismic prediction of karst morphology, with examples from the Loppa High, Norwegian Continental Shelf. This article is awaiting publication and is therefore not included.
Paper 3: Solbakk, Terje; Svånå, Tore Amund; Fichler, Christine. Karst and potential karst on the Norwegian Continental Shelf – basement and basin cover processes and onshore-offshore perspectives. This article is awaiting publication and is therefore not included.