For more than twenty years, the Jurassic succession in the Danish Central Graben has been subject to intense exploration for hydrocarbons. Approximately 43 wild-cats have been drilled and most of these tested structural traps located on footwall crests. The reservoirs encountered were Middle and Upper Jurassic sandstones deposited mainly in nearshore depositional environments. Some of these wells penetrated thin turbidites of Late Jurassic to Early Cretaceous age. Within the Gertrud Graben distinct seismic anomalies indicate the presence of basin floor turbidites, which can be correlated to fan fringe turbidites encountered in the Jeppe-1, Gwen-2 and Mona-1 wells. Seismic inversion provides additional evidence of sandstone deposits in the zone characterized by high acoustic impedance. The turbidites are located within the axial zone of the N W -SE trending Gertrud Graben. Southward the turbidites pinch out gradually; the northern limit of the deposits (in Norwegian territory) is unknown. The deposition of the turbidites is interpreted to be associated with a lowstand of sea-level when conditions were favourable for deposition of coarse-grained deposits in basinal areas. The sediments were probably sourced from an area to the north. Basin axial turbidites are well-known phenomena in various basins. They are not always associated with structural closures and consequently constitute a high risk exploration target. However, in mature basins such as the Danish Central Graben this type of play should be considered in future exploration.
3D seismic data is inverted directly for porosity by implementing a non-linear, globally optimised seismic inversion scheme. The technique takes into account the lithology dependent relationship between porosity and acoustic impedance without requiring the user to make a detailed interpretation of the seismic data and specifying the correlation between acoustic impedance and porosity for each lithological unit. For comparison seismic inversion for acoustic impedance is also carried out. Seismic inversion directly for porosity is implemented by utilising that the correlation between acoustic impedance and porosity has a negative slope which is often approximately constant and not lithology dependent. A wavelet is derived that inverts directly for porosity reflectivity and the absolute level of porosity is defined by constraining to a low frequency porosity model which is specified from available well logs. To invert the seismic data a non-linear globally optimised seismic inversion technique is used. This full 3D multi trace technique does not directly include well logs or seismic horizons and therefore unbiased results are generated from post-stack, migrated seismic data. The seismic inversion is performed by utilising an estimated wavelet, a low frequency model and by specifying 5 inversion parameters. The seismic inversion for both acoustic impedance and porosity is illustrated by a 3D seismic survey. The seismic data was acquired in the Danish part of the Central Trough with the main target being the Chalk Group which includes Upper Cretaceous-Danien chalk reservoirs. 7 wells are used in the study in order to invert a time window from 1.5 to 2.5 sec TWT, that includes a total of 456,000 traces covering an area of 71 sq km. The results show that the technique is a fast and reliable method for estimating porosity directly from seismic data. However, the match between the porosity log and the inversion result is not as good as the correlation between the acoustic impedance log and the corresponding inversion result. This is attributed to the non-unique correlation between acoustic impedance and porosity.
Seismic modelling has been applied to define the porous carbonate intervals and lithologic boundaries resolved in the seismic section at the Løgumkloster‐ 1 Well, southern Jylland, Denmark, This well encountered a 14‐m thick, porous Zechstein carbonate interval (Ca‐2) with an oil show, but the well was plugged and abandoned. Band‐pass filtering and blocking of the acoustic‐impedance log in the time domain have been used to define a number of successive acoustic‐impedance models. The simplest of these, from which a synthetic seismogram correlates with the synthetic seismogram derived from the original acoustic‐impedance log, is assumed to define the maximum number of intervals resolved in the seismic section at the well location. As the well data was acquired in an area represented by a variable seismic reflection pattern, a new optimal seismic section CMP location was found, to optimise the correlation between the synthetic seismogram and the seismic section. Fifteen acoustic‐impedance intervals could be resolved in the seismic section at the Løgumkloster‐1 Well location, including the porous Ca‐2 carbonate interval. A close relationship was found between the Ca‐lb, Ca‐2 and Ca‐3 carbonate layers and lateral variations in the seismic reflection pattern, which indicates significant porosity variations.
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