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Identification of sand injectites in log and seismic data


Stratigraphic sections in sedimentary basins consist of wide range of lithologies. Deposition of clay, particularly where sedimentation rate is high, commonly provides thick stratigraphic units which prevent from dewatering of the underlying section as they have a seal behavior. During burial, fluid pressure increases and might become larger than lithostatic pressure and the fluids will breach the seal. Then reorganization of the sediments resulting from water displacement makes various types of structures ranging from cm-sized contorted- and convolute-bedding to metre- to km-sized network of remobilized sand intruding shaley host rock.

Lessons learned from outcrop analogs indicate that sand injectites consist most commonly of a complex network in 3D of sills and dykes involving a small volume of poorly connected sands likely non-economic, rather than a large volume of porous rock bearing significant producible hydrocarbon reserves. Nevertheless, sand injectites might have contributed to petroleum systems during hydrocarbon charge and also dismigration during geological times.

Both Academia and Industry have paid attention to these large complexes of sand injectites as identification of sedimentary sills and dykes in E&P projects is vital to avoid launching expensive operations such as testing, interference test, completion, water injection and others… Wireline responses of sand injectites look similar to thick stratified sandstone beds with low Gamma-Ray, and commonly a porous response in other logs (DT, NPHI-RHOB…) with fair to good saturation on resistivity logs (So or Sg) if intense diagenesis has not cemented the sand.

A subsurface case study provides a good example of identification in a production well of two “apparently” thick clean sand beds, embedded in a thick shaley section. Top and Base of this shaley section have been interpreted on 3D seismic data. This section shows significant soft deformation and clear evidence of low net to gross as attested by all the other wells drilled through the section. The two sand beds are out of sequence in that shaley section as no thickening-up and no thinning-up trend is observed neither at the base nor at the top respectively. In addition, correlation of all the other wells in the area indicates a lack of sand in that section. At well location, these two sand “beds” overlie a thick confined massive channel fill which is interpreted as the “feeder” for the sand injectites. A complex architecture is anticipated away from the wellbore as suggested by the seismic data and crudely drawn on the seismic line, all outcrop data indicating a high complexity of an almost unpredictable architecture of the injectite network.