Sequence Stratigraphy

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SEQUENCE STRATIGRAPHY OF THE B SAND (UPPER SAND, LOWER GORU FORMATION) IN THE BADIN AREA: IMPLICATIONS FOR DEVELOPMENT AND EXPLOITATIONChris Ebdon, M. Wasimuddin, Arif H. Malik and Shakeel Akhter BP Pakistan Exploration & Production Inc., Islamabad, Pakistan.

ABSTRACT The Badin Blocks operated by BP Pakistan have proved reserves of around 100MMboe, which are currently being produced from over 50 fields. Over 60% of the reserves and production is from the Lower Goru Formation Upper Sand (Early Cretaceous in age), of which the informally named B Sand (part of the Upper Sand, Lower Goru Formation) is the most important reservoir unit. Traditionally the B Sand has been correlated as a lithostratigraphic unit with a sheet like distribution over the area. In places the B Sand has been subdivided into distinctive lower and upper units, but these are not developed everywhere and the controls on the development of these units was not fully understood. A reservoir scale sequence stratigraphic review of the B Sand, based on the integration of core, log and reservoir data, has provided a framework within which the depositional evolution of the B Sand can be explained. The model further highlights implications for field development, particularly reservoir connectivity and the recognition and prediction of persistent stratigraphic barriers, and future exploitation of the Badin Area. INTRODUCTION BP Pakistan operates four separate concessions in the Badin area of the southern Sindh Province of Pakistan. The concession area covers approximately 2,050 square miles. Exploration started in 1977. To the end of 2003, 134 exploration wells have been drilled, making 57 discoveries (Fig. 1). Oil and gas accumulations occur in a complex of Early Cretaceous sandstone reservoirs within the Lower Goru Formation. From top to bottom these are informally termed the Upper, Middle and Basal Sands and are

Aptian-Albian (possibly earliest Cenomanian) in age (Fig. 2). Of these reservoirs, the Upper Sand is economically the most important, hosting 44 discoveries. The Upper Sand itself is further informally divided into three distinct sand units, separated by two regionally extensive mudstones believed to have been deposited during basin margin flooding events. The middle of these sand units is termed the B Sand. It is a high order progradational/ retrogradational cycle 4, 6 bounded below by the Badin Shale and above by the Turk Shale. It is the most prolific of the Upper Sand reservoirs. Historically the B Sand has been correlated as a lithostratigraphic unit across the area. Whilst it was interpreted to have a sheet like distribution, it was also clear that there are significant regional variations in the character and thickness of the B Sand (Fig. 3). The B Sand, where complete, varies in thickness from around 40 to > 300 feet. Local (field wide) stratigraphic compartmentalization was also observed in a number of fields. None of these phenomena could be satisfactorily explained by existing geological models. Detailed studies of core material, integration with wireline log, production and pressure data have provided a sequence stratigraphic model to describe the evolution of the B Sand, explain the regional variations in character and development and predict the presence of barriers and stratigraphic compartmentalization on a local (field wide) scale. REGIONAL SETTING The regional tectonic setting for the Badin area is briefly described by [1]. During the Cretaceous Pakistan and India was part of a northward drifting passive margin, spreading away from the Arabian/African Plate. In the latest Jurassic to Early Cretaceous the Badin area was part of this passive margin (the Thar Platform).


Throughout this time a thick, overall shallowing-upwards clastic sequence was deposited during a series of higher order transgressive/regressive cycles 4. Towards the end of the Albian, at the time of B Sand deposition, a well-established shallow marine shelf was developed. The shoreline was located to the east and south east of the Badin area and deeper water facies towards the northwest. In latest Albian to earliest Cenomanian times a phase of extensional faulting enhanced or imposed the northwest/southeast tectonic grain which provides the tilted fault block structures of the Badin Rift, terminated deposition of shelf sands through rapid deepening and resulted in the development of the regionally extensive Top Lower Goru Unconformity. The Upper Goru Formation mudstones, deposited in deep water, provide a seal to the tilted fault block sandstone plays of the Lower Goru Formation. METHODOLGY The principles of sequence stratigraphy 2, 5 require the recognition of genetically related beds or bed sets (parasequences and parasequence sets) and the key stratigraphic surfaces which bound these genetically related packages 8, 9, 7, 6. The methodology followed in this study starts with the analysis of available core material, which allows the high order identification of individual depositional events, sedimentary structures, bioturbation, trace fossils and interpretation of depositional facies. From successions of facies, parasequences and parasequence sets are recognized and vertical trends in the range of water depths present in successive parasequences are used to identify stacking patterns (Fig. 4). These can also be calibrated with, and interpreted from, wireline log data. Key stratigraphic surfaces are also identified in core. Candidate sequence boundaries are identified based on one or more of the following criteria: clearly defined erosional truncation, direct evidence of sub aerial exposure, or abrupt basinward shifts of facies. Likewise, potential condensed sections should be recognized based on unusual burrowed surfaces, abundant diagenetic minerals, fossil concentrations, closely spaced bentonite beds, or radioactive shales. Condensed sections may, but do not necessarily, lie along the maximum flooding surface. During the course of this study, cores from ten B Sand wells have been examined and core photographs from a further eight reviewed. Wireline log data from in excess of another 100 wells have also been reviewed and incorporated. From the recognition of parasequence sets and key stratigraphic surfaces in cores and on wireline log data the B Sand can broadly be divided

into systems tracts, which have been correlated and mapped over the Badin area. B SAND SEQUENCE STRATIGRAPHIC EVOLUTION: KEY SURFACES AND SYSTEMS TRACTS The Badin Shale Maximum Flooding Surface The Badin Shale, where not truncated or removed by the top Lower Goru unconformity, is well developed over the Badin area, varying in thickness from 50 to >120 feet. The unit is characteristically developed as a mudstone facies. Core material is available from the Badin Shale in only one well, Duphri 2, where it is represented by finely laminated, occasionally bioturbated mudstones interpreted as offshore marine mudstones deposited well below storm wave base. The maximum flooding surface itself, however, is not penetrated by core. On wireline log data a maximum flooding surface is interpreted within the Badin Shale at a gamma maximum (e.g. Zaur 9; Fig. 5, Duphri 2; (Fig. 6). Tangri 8; Fig. 7) which represents concentration of radioactive minerals during sediment starvation. More calcareous rich horizons are also associated with this event, which are consistent with a reduction of clastic depositional rate. Occasionally it can be difficult to confidently pick the exact location of the maximum flooding surface within a 10-20 feet thick zone of maximum flooding. However, on logs an overall fining upwards trend is recognized below the maximum flooding surface (or zone of maximum flooding), and at a finer scale retrogradational parasequence sets can be interpreted (e.g. Tangri 8, (Fig. 7), from 7500 feet to MFS). Above the maximum flooding surface there is an overall coarsening upwards profile and stacked progradational parasequence sets can be interpreted (e.g. Zaur 9, Fig. 5). The B Sand Highstand Systems Tract (HST) The HST represents a period of shelf outbuilding and progradation during a period of relatively stable sealevel following the maximum flooding surface (Fig. 4). In the Badin area core is available from the HST in a number of wells. The best coverage is from well Zaur 9 (Fig. 5), which is selected as the type well. Here one 60 foot core is available, the majority of which is assigned to the HST. The sandstones encountered are variably argillaceous, locally bioclastic and are typically fine grained and well sorted and form a subtle coarsening upwards succession. Bioclastic material is common in the lower parts of the core and the amount of bioclastic material and degree of bioturbation broadly reduce upwards. At the base of


the core the sandstones are intensely bioturbated and variably bioclastic. Bioclasts are dominated by bivalves and fragmented shell material but also include large calcareous benthonic foraminifera and rare coral fragments. These units are sharp based and are interpreted as storm event beds (Fig.5 E). They pass upwards into bioclastic cross bedded sandstones and ripple laminated sandstones with restricted ichnofabrics which suggest a shallowing upwards succession within an outer shelf setting. Within the core at 6576 feet (drilled depth) a candidate parasequence boundary or high order flooding surface is identified based on an abrupt increase in bioturbation index 3 and slightly finer grained and more argillaceous sediments. This parasequence boundary is also expressed on wireline logs as a gamma peak (Fig. 5 D). Above this parasequence boundary similar facies to those observed at the base of the core are again observed, consistent with the deposition of storm beds within the lower shoreface/offshore transition zone. These intensely bioturbated sandstones occasionally show evidence of the deve