Neogene Biostratigraphy and Sea Level Changes — Case Study from North Kuwait Field
Mishra, Prasanta K.*1; Akbar, Bibi H.1; Ahmed, Khalid 1; Choudhary, Pradeep K.1; Hassan, Ferdous 1; Packer, Stephen 2; Starkie, Stephen P.2; Stead, Darrin T.2
(1) KUWAIT OIL COMPANY, Ahmadi, Kuwait. (2) Millenia Stratigraphic Consultants, Buckinghamshire, United Kingdom.
The Neogene clastics (sandstones and shale interventions) in one of the north Kuwait fields range in age from Early Miocene (Burdigalian) to Middle Miocene (Serravalian). An attempt was made to carry out detailed biostratigraphy (foraminifera, nannoplankton, palynomorphs and dinoflagellate cysts) from study of selected core samples in this geological succession and establish the sea level changes and depositional environment leading to sequence stratigraphic interpretations.
The bottom most sands are devoid of any microfossils and inferred to be deposited in a fluvial channel facies. The first basal transgression corresponds to the prominent intermediate Shale layer between two reservoir sands and an abundance of Dinocysts, influx of Pediastrum sp was observed. The sediments are inferred to have been deposited in an inner neritic regime. This is followed by a swallowing trend of sea level in the immediate upper layers grading towards transitional/non marine conditions.
The definite indications of Early Miocene (Burdigalian) transgression are evidenced from microfossils from the first Shale with the record of H.Cincutm and palynomorphs viz. S.ramosus and smaller benthic foraminifera. A large influx of benthic foraminifers’ viz. Dendritina sp indicated an increase in sea level from up to the Middle Neritic conditions.
The topmost prominent transgressive succession prevailed over is the Serravalian (Middle Miocene), corresponding to the upper most Shale succession. The bottom part of this thick shale layer is rich in benthic foraminifera, Miogypsinids, rare planktics viz.Globigerina sp,G.priamenardii and nanno plankton species, Spenolithus heteromorphus and a strong influx of dinoflagellate cysts. A middle neritic bathymetry was inferred at the bottom of the upper shale which gradually shallowed to inner neritic conditions at the top of this shale. Thus the Maximum flooding surface was inferred at the bottom of the upper most shale. Fresh water influx was inferred at the top of the upper shale as supported from the occurrence of diatoms, Bottryoccous pediastrum and ferruginous layers in the cores.
The reservoir sands are more or less distributed in between the transgressive episodes in an inter-distributary bay /lagoonal deposits. An integration with the electrolog and sedimentologcal inputs was attempted to decipher the variation of reservoir facies in the study area.
AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain