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Volpriehausen prospectivity Review (2014)

This MSc thesis has been written by former intern Mariska van Eijk.

The Volpriehausen Sandstone constitutes the second largest play in the Netherlands with onshore and offshore gas fields. Many Volpriehausen exploration wells in the study area in the Step Graben and Central Graben are old and based on 2D seismic. Now, with large 3D seismic coverage available, the structures and the amplitude support, which is typically used in exploration of the Volpriehausen Formation, is remapped and checked in the study area, in the northern Dutch offshore.

Gas fields in the Volpriehausen are not always clearly characterized by a structural conformable amplitude anomaly, as appears from an evaluation of Volpriehausen gas fields. Also, no standard seismic character can be determined for a gas-filled Volpriehausen reservoir, since amplitudes of the top high amplitude reflector differ between the evaluated fields. Even though all evaluated gas fields have bright spots, these are not fully reliable direct hydrocarbon indicators (DHI’s) since the reflector strength can also be influenced by other factors than gas fill, including salt plugging, porosity, tuning, processing effects and lithology transitions. Seismic interpretation, a dry well analysis and a comparison with Volpriehausen gas fields resulted in a better understanding about the controls on hydrocarbon distribution in the Volpriehausen Formation in the northern Dutch offshore. The Volpriehausen reservoir is generally present in consistent thickness (although some thinning to the north occurs); locally the reservoir is absent due to salt diapirism. Reservoir quality depends on the porosity, permeability, diagenesis and clay-content. The reservoir potential generally decreases towards the north, due to a decreasing thickness (from 70 to 15 m) and increasing clay-content in this direction (from <5 to 32%), consistent with the depositional model of a fluvial system building out northwards. The porosity varies between 14-28%, but a trend is difficult to recognize, due to limited well availability and inconsistent porosity calculations across the wells. Salt plugging of the pores decreases the reservoir quality, but might also provide a side-seal for the rest of the reservoir, as seen in the M1-A field. Halokinesis is important for trap formation, forming turtle-back anticlines, 3-way dip-closures against salt walls and 4-way dip-closures above salt structures, and may also reactivate faults, creating charge windows in the underlying Zechstein salt, as seen near wells E09-02, F04-03 and G07-02.

The overlying Volpriehausen Claystone Member acts as top-seal in the entire study area, except at unconformities (E09, F10), where the sealing formation is thin or absent due to erosion. At these unconformities the Lower Cretaceous Shale forms the top-seal, additionally to the Volpriehausen Claystone Member. Faults in the overlying Claystone Member provide possible migration paths for hydrocarbons out of the Volpriehausen Sandstone Member, which has possibly happened in the four-way dip-closures of F04-01 and F04-03. Although, another more likely explanation for these dry holes is the absence of hydrocarbon migration into the trap. Charge forms the largest risk for the Volpriehausen play in the study area. The F15-A field is the northernmost Volpriehausen gas field and much is unknown or uncertain about charge into the Volpriehausen north of F15. Migration is difficult, since Slochteren shales, Zechstein Claystones, Zechstein salt and the Main Claystone Member (a.k.a. Lower Bunter shales) need to be crossed. Faults and ‘withdrawn’ Zechstein may provide migration paths. Differences in maturity of the source rocks are expected to exist between the Step Graben and Dutch Central Graben, since the DCG experienced larger amounts of burial. Dry hole analysis has shown that out of 20 wells, 12 found Volpriehausen reservoir. Of those 12, 4 are analyzed to be a valid negative test, most likely due to lack of hydrocarbon migration. All of these are situated in the Step Graben, leaving the Central Graben void of any valid Volpriehausen tests. The other 8 are not drilled within a Volpriehausen closure or have updip closure (>25 m). Analysis of seismic and well data has resulted in a prospect inventory, of which the Hutton, Cuvier, Ziegler, Wegener, Lyell, Anning and Kingfisher leads are considered to be most promising. Charge is considered to be the main risk for these leads. Although, in the G07-02 well near the Hutton lead trace gas has been encountered and the presence of charge is therefore considered less of a risk for this lead. For the Kingfisher, Ziegler and Anning leads overmaturity of the source rocks forms an additional risk, since they are located in the Dutch Central Graben, which experienced a large amount of burial.