Diagenetic Exaggeration of Primary Texural Variations in Incised Valley Fills, Cusiana Field, Colombia
Edward Warren and Andy Pulham
Incised Valley Fills in the late Eocene Mirador Formation of the Cusiana Field are dominated by quartz arenite sandstones that vary in grain size from fine sandstones to sandy conglomerates. The average porosity of the valley sandstones is ~8% which reflects significant compaction during deep burial (~20,000 feet) and abundant quartz cement (~14%) precipitated over a wide temperature range (85-140°C).
Surprisingly, the present permeability structure of the valley fill sandstones still reflects grain size variations. Indeed, the effect of the large amounts of quartz cement has been to exaggerate the primary textural variation in permeability. Very coarse, well sorted sandstones at the valley bases possess two orders of magnitude higher permeability than fine-grained equivalents of the same porosity. This permeability contrast is more than original depositional textures in sandstones would predict and is consistent with predictions from simple sphere pack models which show that at low porosity (<12%), the contrast in permeability due to grain size is greatly enhanced.
As a result the reservoir characterization has been based on the primary sedimentary fabric, not on diagenesis. Production logging results show that the coarse grained bases of the valley fills are indeed the major contributors to flow. Thus the impact of diagenesis on the flow structure of these valley fills has been to selectively reduce the permeability of finer-grained sandstones, while maintaining high permeability in the coarsest grained bases of the valley fills.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California