EAPG/AAPG Special Conference on Chalk

Stratigraphic studies of Late Cretaceous sequences in northwest Europe frequently fail to
identify subtIe changes in both lithofacies and biofacies due to the apparent monotonous nature
of chalk sediments. This results in the non-recognition of many major factors affecting the
stratigraphic succession, well geohistories and the mis-interpretation of regional sequentiaI
history.

A key exercise for planning aspects associated with production of petroleum reservoirs is
modelling and simulation. The adequacy of such simulations depends on good reservoir
characterisations, i.e. good estimates of geological, petrophysical and multiphase flow
properties of reservoir rocks and the reservoir structure as a whole.

Biostratigraphical studies of the Late Cretaceous Chalks in North
West European onshore sections have traditionally concentrated
on inception and extinction events and the age dating of
individual outcrop localities.

For carrying out stochastic modelling of reservoir sequences, information on the
correlation structure of the properties modelled is needed. The correlation structure
is usually quantified with variograms describing the correlation ranges in different
directions.

Fractures are present in almost all chalk reservoirs, but it is only
when these fractures from an interconnectrd network that their affect on
fluid flow becomes important. Fractures not only enhance the overall
permeability of many reservoirs, they create significant permeability anisotropy.

Silo field, Wyoming, was the original site of research by the Reservoir Characterization Project
(RCP) at Colorado School of Mines into the application of multicomponent 3-D seismic surveys to
characterize the fractured chalk reservoir. These surveys, the first of their kind, resulted in the
accurate prediction of the dominant open fracture direction and the location of enhanced fracture
permeability zones in the reservoir. Results of the multicomponent 3-D seismic program at Silo field
led to horizontal drilling in the field which has confirmed the seismic characterization (Figure 1).

In the late 1980'ies it was discovered on seismic data and
synthetic seismograms that the seismic phase of the Top Chalk
reflection response changed from the crestal gas zone to the waterbearing
flanks.

The Tor Field was discovered in late 1970. First production commenced in June, 1978 with 5 wells drilled from
a fixed 18 slot platform (Figure 1). After additional development drilling, peak oil production of 100,000 BOPD
was achieved in 1980, but has since declined to the current rate of 6,000 BOPD.

The Upper Cretaceous Austin Chalk produces along the Gulf of Mexico coast, United
States, from secondary porosity and permeability created through fracturing. Several large fields,
including Giddings (222 MMBO, 76 BCF) and Pearsall (104 MMBO, 15 BCF) were developed
utilizing mostly vertical wells.

The deformation of high porosity chalks has become a major factor in understanding their performance as
hydrocarbon reservoirs. Chalks with porosities in excess of 30% readily undergo large volumetric strains when
subject to fluid production and pore pressure depletion in many cases passing through a phase transition from
elastic to ductile compaction behaviour with commensurate large increases in compressibility.