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Identifying Overlooked Exploration Opportunities from Bypassed Pay Analysis (2014)

This MSc thesis has been written by former intern Manuel Bussmann.

Bypassed pay analyses may reveal potentially overlooked exploration opportunities, as recognized by Lutgert et al. (2013). Analysis of a comprehensive petrophysical database reveals the presence of several overlooked exploration opportunities in the Broad Fourteens Basin, West Netherlands Basin and Roer Valley Graben. Untested hydrocarbon potential is recognized in the Chalk, Holland Greensand, Delfland, Brabant Limestone, Middle Werkendam, Lower Muschelkalk, Zechstein Fringe sandstones + carbonates and Westphalian C/D.

In particular, the understudied Middle Jurassic Brabant Formation is an excellent example of an overlooked exploration opportunity. Untested good-quality oil shows are found in more than 10 wells in the Roer Valley Graben and West Netherlands Basin. The main objectives of this study are twofold: 1) to create a conceptual geological model for the Brabant Formation, that can be used to understand and predict reservoir presence and quality on a first-order basis, and 2) to identify trap concepts. This study integrates seismic, well log and core data in combination with study of analogue formations from literature.

The Brabant Formation of the Altena Group (Bathonian – Oxfordian) comprises three re- and transgressive cycles of sandy limestone – marl deposition with an oolitic limestone on top. The present-day distribution of the Brabant Formation is, as a result of uplift and erosion, confined to the Roer Valley Graben, West Netherlands Basin and small areas in the Broad Fourteens Basin and Central Netherlands Basin. The formation is here interpreted to have been deposited on a shallow marine, transport-dominated carbonate ramp with gentle depositional slope (< 0.1º). Consequently, facies belts are wide, lateral facies changes are subtle and vertical facies changes are rapid. This has resulted in “layer-cake”-like stratification in a predominantly aggradational facies architecture. The regressive intervals have reservoir potential, whereas the transgressive intervals have seal potential for oil. Three depositional environments are recognized from cores: upper shoreface or shoal facies (inner ramp), lower shoreface facies (mid ramp) and offshore facies (outer ramp). Sandy calcarenitic and bioclastic grainstones of upper shoreface facies are clearly the most reservoir-prone (ϕAvg = 11%, KAvg = 10 mD). Storm-influenced marls and calcareous siltstones of lower shoreface facies are potential waste zones with intermediate reservoir and seal properties. Offshore silty marls and claystones have seal potential for oil (ϕAvg = 8%, KAvg = 0.7 mD) as proven in wells Andel-1 and Lekkerkerk-1. Gross reservoir thickness increases towards former depocentres (Roer Valley Graben, West Netherlands Basin) and is likely at a maximum in fastest subsiding grabens. In contrast, basin margin sequences are thinner and more amalgamated. During syn-tectonic deposition, grabens accumulated thicker sedimentary sequences, most likely still in similar facies. Thus, more potential net pay is expected in areas of higher subsidence (grabens). The area with highest subsidence, highest stacked reservoir potential (i.e. presence of ATBR1, ATBR2 and ATBR3) and most reservoir-prone facies is found in the northwest Roer Valley Graben and border with the West Netherlands Basin. It is speculated that throughout the Middle Jurassic this area was a large shoal, notably shallower than the basin centers of the West Netherlands Basin and Roer Valley Graben. Towards the basin center of the West Netherlands Basin, reservoir quality deteriorates slightly as a result of subtle change to more distal facies. This lateral facies change trend could not be confidently confirmed in the Roer Valley Graben due to lack of well data, but future drilling activities in this area have been planned and may help to understand this.

The Brabant Formation play is proven with two stranded oil discoveries with reservoirs at Brabant level: Andel and Lekkerkerk. Potential is further indicated by good-quality oil shows encountered in more than 10 wells in the Roer Valley Graben and West Netherlands Basin. In this area, prospectivity is demonstrated on several representative seismic sections. Four prospective trap configurations at Brabant level have been identified: fault-dip closures, downfaulted traps, inversion anticlines and sub-unconformity traps. Downfaulted and sub-unconformity traps are undrilled. Risks are primarily prospect-dependent and mainly related to charge and seal. Prospectivity in the Broad Fourteens Basin was not studied and remains speculative, but it is recognized that the formation may be locally truncated here against Vlieland Claystones acting as a seal. Future mapping of the formation will likely result in prospect identification and upgrades the hydrocarbon prospectivity of the Roer Valley Graben, West Netherlands Basin and, possibly, Broad Fourteens Basin.