Origin of Formation Water Salinity Variation in Ultra-Low Permeability Clastic Reservoirs

Abstract

The origin of formation water salinity variation, in the bottom of upper Triassic, the northwest of Ordos basin, China, is studied. 91 formation water samples show that water salinity is characterized by a wide range (5g/L–95g/L) and a complex plane distribution. In order to find out the main cause of such distribution complexity, core data, scanning electron microscope(SEM) images, chemical analysis results of formation water and log data are deeply analyzed from perspectives of diagenesis and tectonism. And then, their characteristics are presented as the followings. In high salinity area, tuff is found growing through SEM images and well log response characteristics. Meanwhile, when total salinity is higher than 40g/L, the condition of Ca2+ is opposite to that of Na++K+, for its increasing rate of concentration increases with total salinity accumulating. It means that besides albitization of plagioclase, some other factors should contribute to such abnormal phenomenon. In low salinity area, while, with fracture and faults developing, water type changes from CaCl2 to MgCl2, NaHCO3 or Na2SO4 type. Thus, the main causes are proposed to be composed of two aspects. One covers tuff alteration and later diagenesis for the high salinity. To be specific, montmorillonite, developed from tuff alteration, absorbs Ca2+ from formation water selectively and then ions migrate, during which more Na++K+ get lost, while more Ca2+ reserved. Afterwards, those reserved Ca2+ get released with montmorillonite transforming to illite, which results in a loss of Na++K+ and accumulation of Ca2+. Lots of ions are released into formation water during that process and later diagenetic process, which leads to the high water salinity. The other aspect is the development of faults and fractures, through which, formation water becomes the mixture of connate water and upper low salinity water. And that is the main cause of low salinity. The results reveal that in any other analogous clastic reservoirs, formation water salinity probably varies significantly because of complex diagenesis and tectonism. It is necessary to use variable formation water salinity in formation evaluation.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015