Study of High Resolution Sequence Stratigraphy and Reservoir Characteristics of Khuff Formation in South Pars Gas Field
The geological features of South Pars Gas Field are basically clear. Tectonically, it is located in the northeast of South Pars Anticline. The target zones are the upper Permian - lower Triassic Khuff Formation marine carbonate and evaporite strata. The lithology of the reservoir is limestone and dolomite. The reservoir type is predominantly pore type and under developed fractures. Horizontal distribution of the reservoir is relatively stable at the main position of the north - south Pars Anticline; however, the heterogeneity of reservoir strengthens is at the pitching end of the anticline. Therefore, in allusion to the problems like reservoir heterogeneity, this study aims to make full use of geologic and geophysical data to make a scrutiny into the gas field, high resolution sequence stratigraphy, deposition, and reservoir characteristic have been conducted. Firstly, based on the detailed observation on cores and slices as well as the integrated analysis on logs and mud logging data, the following aforesaid clue is combined with the facies sequence analysis. The Khuff Formation is divided into seven depositional sequences (KQ7, KQ6, KQ5, KQ4, KQ3, KQ2, and KQ1) which contain eleven system tracts and fifty four level V. Secondly, according to the study, it is shown by research findings of stratigraphic deposition and reservoir characteristics that the reservoir property characteristics of the gas reservoir is mainly controlled by sedimentary facies. The reservoir is mostly developed in the KQ6 sequence and in the KQ3 sequence interval where the coarse grain lithofacies deposition (barrier beach facies belt) predominate. And these sequence comprises a series of generally fining upwards cycles reflecting shallowing from foreshoal mudstones (transgressive), through well developed, high energy coarser grained barrier shoal sediments (aggrading highstand) to less well developed tidal flat and lagoonal sediments. The reservoir space is predominantly the secondary dissolution pore and small solution cavity resulted from denudation. Thirdly, under the direction of the original stratigraphic division scheme, the difference of longitudinal reservoir characteristics is pointed out in this study.
KQ7 Depositional Sequence
The sequence is entirely dolomitize and is dominated by peloidal and ooid packstones, grainstones, mudstones, wackestones, and boundstones lithofacies. The sequence comprises a series of generally fining upwards cycles reflecting shallowing from foreshoal mudstones, through high energy coarser grained barrier shoal sediments to finer tidal flat and lagoonal sediments. This is typical of the depositional units, which demonstrate further 6 shallowing events.
The base of the sequence is dominated by dolomitize mudstones, relict grain grainstones, and relatively thick sequences of displacive anhydrite. The middle part of the sequence predominantly comprises grain dominated in thinly bedded units (~0.5 m) with common pebbly basal lags. The uppermost part of KQ7 is dominated by dolomitize rootlet/fenestral mudstones and dolomitize laminated stromatolitic boundstones with rare thrombolitic boundstones.
KQ6 Depositional Sequence
The base of the sequence is dominated by dolomitize boundstones and mudstones derived from both relatively shallow and relatively deep environments. The majority of the depositional sequence predominantly comprises thick intervals of ooid or peloidal and coated grain dominated lithofacies with little variation in grain size except for a tendency to fine upwards and become muddier at the top of each individual unit. These mud supported intervals become thicker upwards. The oolitic and peloidal intervals are commonly massive, but may also exhibit planar to low angle lamination with rare trough cross-bedding and current ripples.
The uppermost part is dominated by mudstones and wackestones with common planar lamination, horizontal bioturbation, abundant miliolids, variably bioturbated and laminated peloidal, and peloidal intraclastic packstones and grainstones.
KQ5 Depositional Sequence
This sequence is characterized by a high proportion of mudstones and wackestones with infrequent anhydrite intervals. Peloidal, peloidal intraclastic, and relict grain packstones and grainstones are only thinly developed, interbedded with the thick mudstone intervals. The sequence comprises a series of variable thickness, fining upwards cycles reflecting shallowing from foreshoal mudstones, through poorly developed high energy coarser grained barrier shoal sediments to finer, but well developed lagoonal, tidal flat and evaporitic pool/sabkha sediments.
KQ4 Depositional Sequence
This sequence is dominated by peloidal and bioclastic packstones and grainstones intercalated with mudstones and wackestones. The KQ4 sequence can be split into a lower and an upper interval, representing two large sub-cycles. The lower interval consists of a series of stacked, generally fining upwards sediments dominated by dolomitize mudstones and wackestones interbedded with relatively thin packstones and grainstones.
The upper interval comprises a stacked series of thinly interbedded, generally fining upwards peloidal packstones, grainstones, mudstones, and wackestones. The mudstones, wackestones, packstones, and grainstones are generally massive, with only rare lamination or bioturbation.
KQ3 Depositional Sequence
This sequence is dominated by peloidal and bioclastic packstones and grainstones intercalated with mudstones and wackestones (MWd). The KQ3 depositional sequence can be split into a lower and an upper interval.
KQ2 Depositional Sequence
KQ2 depositional sequence shows a series of short, thin, generally shallowing upwards sequences, from middle shelf foreshoal to tidal flat facies association.
This depositional sequence is dominated by thinly interbedded boundstones, mudstones, wackestones and peloidal, ooid, ooid intraclastic, bioclastic packstones, and grainstones. The top of this unit is marked by a sharp erosional flooding surface and the deposition of transgressive barrier shoal grainstones. And the base of this sequence is marked by a flood back and facies shift to foreshoal environment followed by a shallowing to shoal and tidal flats. Minor transgression resulted in a return to dominant barrier shoal sedimentation with rare intertidal flat or lagoon development.
KQ1 Depositional Sequence
This sequence is dominated by thinly interbedded mudstones and wackestones and peloidal and ooid packstones and grainstones with thin laminated boundstones. The base of this unit comprises thinly interbedded, strongly laminated mudstones, and variably laminated and locally current rippled peloidal and ooid grainstones at the base. And the top of the unit is marked by a transgression with shoal sedimentation shallowing up to a tidal flat to the end of the first of two small scale depositional cycles.The barrier beach facies is the predominant facies type in the study area, and the oolitic beach and bioclastic beach microfacies is the favorable reservoir facies belt, where coarse deposit rocks like oolitic limestone, granula limestone, and a few pelleted limestone mainly developed. The reservoir mostly developed in the KQ6 sequence and in the KQ3 sequence interval where the coarse grain lithofacies deposition (barrier beach facies belt) predominate. And these sequence comprises a series of generally fining upwards cycles reflecting shallowing from foreshoal mudstones (transgressive), through well developed, high energy coarser grained barrier shoal sediments (aggrading highstand) to less well developed tidal flat and lagoonal sediments. The reservoir space is predominantly the secondary dissolution pore and small solution cavity resulted from denudation.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California