What’s new in exploration

 Vol. 232 No. 9

The Dutch North Sea is a mature area, with many conventional fields developed over the past 50 years and other discoveries left temporarily stranded. Shallow gas accumulations on the shelf were recognized in the early 1970s but were never developed because of low reservoir pressures and unconsolidated sand issues. The potential of some offshore licenses was explored more recently by Nederlandse Aardolie Maatschappij (NAM), a joint venture of Royal Dutch Shell and ExxonMobil.

Many operators have interest in the North Sea’s shallow gas, and some are pooling data in a joint industry project underway at the non-profit TNO, the Netherlands Organization for Applied Scientific Research (www.tno.nl).

A12. The first project to produce from shallow accumulations is Chevron’s A12, in about 100-ft water depth on the Dutch shelf. Unocal worked the Dutch A and B quadrants acquired from NAM in 2004, which passed to Chevron in the 2005 merger. Chevron and its partners Dyas, DSM Energie and state-owned Energie Beheer Nederland (EBN) began producing from stacked pay horizons at A12 field on Dec. 20, 2007. Success at A12 led to a resurgence in shallow gas interest, particularly in the northern offshore sector (A and B blocks).

Dutch shelf. Shallow gas occurs in unconsolidated, Cenozoic sediments on the Dutch continental shelf, particularly in shallow marine to continental (deltaic) deposits of the Plio-Pleistocene Eridanos delta, in Pleistocene tunnel-valley fill deposits and in volcaniclastics at the base of the Paleocene (e.g., Basal Dongen tuffite).

The shallow marine to continental deposits accumulated both from deep thermogenic sources and from biogenic sources in shallower strata. They are either structurally trapped in low-relief anticlines associated with rising salt domes or occur in stratigraphic or depositional traps. The gas accumulations may be economically attractive, especially if located near existing infrastructures. The Dutch thermogenic, biogenic and mixed shallow gas systems are highly dynamic. Gas chimneys, acoustic turbidity and blanking, pockmarks and stacked bright spots, as well as methane dissolved in groundwater, all indicate ongoing and/or recent migration and leakage of shallow gas.

Shallow gas production in the Dutch offshore is still limited by a lack of familiarity with the shallow gas systems, especially with respect to the relationship between anatomy of the delta and charging/trapping conditions. TNO personnel believe there is a relationship, though still unclear, between the occurrence of shallow gas (bright spots) and the depositional environment. In fact, previous studies suggested the bright spots are related to specific stratigraphic intervals.1

Direct hydrocarbon indicators for tunnel valleys include bright spots and several other types of velocity anomalies, which are well known by offshore operators as they may strongly affect the correct imaging of deeper subsurface (energy absorption, ray distortion, etc). Their study and characterization have important industry applications, both in geohazard identification and in exploration of deeper levels.

E&P would clearly benefit from an increased understanding of how the distribution of these environments is related to external (climate, tectonics, sea-level) and internal (delta-lobe switching, avulsion) processes. A sequence-stratigraphic framework should provide critical information about the architecture and reservoir properties of these not yet well-understood reservoirs and the continuity of sealing clays.

The established reservoir model—in combination with reconstruction of the areas of origin of biogenic and thermogenic gases and the charging, preservation and pressure conditions in these reservoirs—forms the basis for improved prediction of other potential shallow gas occurrences offshore the Netherlands.

TNO’s geology and petroleum research center at Utrecht University in the Netherlands.

Current JIP. The shallow-gas JIP at TNO is sponsored by EBN and four other participants, and focuses on the architecture of the Eridanos delta and the occurrence of shallow gas. According to Dr. Elisa Guasti at TNO, the project began this spring and will last about 18 months. The aim is to develop a 3D, basin-scale reservoir model of the shallow gas systems contained in the Cenozoic Eridanos delta. Key external controls will be linked directly to depositional elements (fans, valley fills, clinoforms and topsets) calibrated by extensive well data, providing bio- and chronostratigraphic, geochemical and paleoenvironmental information.

This will form the basis for a migration/charging model based on present-day fluid dynamic data and basin modeling to predict shallow gas occurrences, delineating economic profitability and hazard potential. The main result of the study will be a workflow that assesses the potential of seismic bright spots as indicators for shallow gas occurrences.  

LITERATURE CITED
1 Kuhlmann, G. and T. E. Wong, “Pliocene paleoenvironment evolution as interpreted from 3D-seismic data in the southern North Sea, Dutch offshore sector,” Marine and Petroleum Geology, 25, No. 2, 2008, pp. 173–189.