Marianne Rauch-Davies¹ and Alex Berkovitch¹
¹Geomage 2003 Ltd
Unconventional reservoirs are being explored using conventional exploration methods but successful drilling within these reservoirs has a unique set of problems. Most wells are drilled horizontally through the reservoir rock and fracking is applied to generate permeability and produce hydrocarbons. The pre-drill knowledge of natural fracture swarms and small offset faults is very important as these geological elements can either help increasing the flow rate or can interfere with the drilling and fracking process and negatively influence the production rate. Seismic resolution from conventional reflection imaging is generally not sufficient to resolve these small scale rock properties. Diffracted waves are events that are produced by the scattering of a wave after it meets a discontinuity such as fracture swarms, small amplitude faults and karsts that cause local sharp changes in the geometrical or lithological characteristics.
The MultiFocusing diffraction imaging technology uses the MultiFocusing concept to separate the diffraction component from the overall wave field. The amplitudes of diffracted waves are usually much weaker than those of specular reflections. Diffractions are essentially lost during the conventional processing/migration sequence, or they are masked in conventional seismic stacked sections. Local structural and lithological elements in the subsurface of a size comparable to the wavelength are commonly ignored during processing and identified indirectly during interpretation but not from direct measurements. Diffractions are the only measurements that allow us to directly map small scale heterogeneities in the subsurface from pre-existing seismic data.
We are able to show results of diffraction studies over 3 distinctly different study areas. The first project is located in the Levant Basin, Mediterranean and involves diffraction imaging of 2D data. This region has again gained interest as large gas deposits were recently discovered. Clastic deposits are overlain Messinian evaporites with the buildup of anhydrates at the top of the salts and just below the reservoir. These anhydrates are uneven in appearance and are expected to produce strong diffraction anomalies. Furthermore, more or less vertical faulting is characteristic for this play and hard to detect using reflection seismic.
The second example that we are presenting is from the Upper Jurassic Bazhenov Formation in Siberia. This large scale deposit has sourced prolific deposits in the shallower part of the Siberian Basin for over 20 years but has gained importance as main reservoir layer since the advance of horizontal drilling and fracking technologies. Fracturing within the Bazhenov occurred when hydrocarbons were generated and the pressure associated with this process was larger than the lithostatic pressure. As such it is important to predict these geological small scale features pre-drilling. Diffraction imaging was performed on a 3D dataset and integrated and calibrated to other data types such as well information etc.
Our final study discusses diffraction imaging for fracture detection in an unconventional formation in North America. This reservoir unit is composed of black shale in the lower part, and siltstone and limestone in the upper part. It is between 7 and 60 m thick and is at a depth between 1000 and 2000 m and it is rich on organic matter and better flow rates are achieved in areas with natural fracturing. A 3D dataset was available and the diffraction results were calibrated to other data types like, surface condition, well information and the conventional PSTM.
AAPG Datapages/Search and Discovery Article #90190©AAPG Southwest Section Annual Convention, Midland, Texas, May 11-14, 2014