Al Khlata Oman

The purpose of the palynological subdivision described here is to correlate and describe the Al Khlata Formation in the Mukhaizna area, Oman. A number of boreholes were drilled in 2005 and 2006 to analyse stratigraphy; and for this study comprehensive suites of cuttings samples and a few sidewall core samples from these wells were combined with samples from earlier wells, including mainly cuttings and sidewall core samples but also very limited sections of core. Four hundred samples formed the database of the study. The lack of core and sidewall core samples necessitated a different approach to that traditionally used in subsurface Al Khlata Formation palynostratigraphy (e.g. Penney et al. 2008), which relies mainly on quantitative characterisation (from core and sidewall core data) of assemblages by assignment of palynomorphs to a small number of suprageneric groups, for example the ‘Horriditriletes Group’ and the ‘Microbaculispora Group’. With such a scheme, there are problems assigning assemblages from cuttings samples. An example is the distinction of the 2159 and 2165 biozones of PDO (Penney et al. 2008). In brief, these biozones are defined by the presence of different proportions of monosaccate pollen, Punctatisporites, cavate-zonate spores and Anapiculatisporites concinnus (Penney et al. 2008). In core or sidewall core samples such distinctions can be very effective because assemblages can be assumed to be in situ. However, in cuttings samples, all but one of the palynomorph groups or taxa that help to define 2159 and 2165 biozones are likely to have caved from sections above (in which they are common). Thus the proportions of palynomorph groups or taxa in cuttings samples from the lower part of the Al Khlata Formation might be entirely artificial, and are therefore a poor basis for biozonal subdivision. However there are distinct species which have restricted ranges within the lower parts of the Al Khlata Formation, such as Spelaeotriletes triangulus and Ahrensisoprites cristatus and these are important because if they are present in a cuttings sample, they cannot have caved from above. The first downhole appearances (FDAs) of such taxa form datums that can be used as an alternative to those based on quantitative character change in assemblages which are susceptible to caving, thus they are key to downhole biozonation based on cuttings. Read more - Stephenson, M. H., A. Al Rawahi and B. Casey 2008. Correlation of the Al Khlata Formation in the Mukhaizna Field, Oman, based on a new downhole, cuttings-based palynostratigraphic scheme. GeoArabia, 13, 45-75.