Geo pressure signify potential hydrocarbon reserve in offshore Bangladesh

Geo pressure signify potential hydrocarbon reserve

Petroleum is leading energy source and will dominant as prime source for next 2-3 decades. After 2050 petroleum may loss it’s top position as raw energy source and may decline consumption of petroleum all over the world. Consequently petroleum will no loger be the primary object of energy generation and will reduce the worldwide hydrocarbon exploration activities. Only for late decision of offshore exploration it may remain unexplore vast offshore area of Bangladesh, whilst the offshore drlling technology will be insufficient and costly in future.  

 

Geologically Bangladesh is located at mouth of active Bengal delta and in the subduction zone of Burmese plate & Indian plate. Hydrocarbon reserve in this deltaic region have great effect of plate tectonic movement, very rapid & juvenile sedimentation process and undeformed compaction of sediment. For hydrocarbon exploration drilling is ultimate method for extraction of oil & gas from reservior and it is most important to analyse subsurface formation, related positive or negative geopressure anomalies and ultimate status of petroleum reservoir.

 

It is now possible to view the petroleum province within an overall geodynamic picture resulting from an integration of structural deformations, sedimentary changes, heat flow, fluid transfer or migration etc. In fact all are intimately linked and a variation in any one parameter acts on all others, constitute the petroleum system.

 

Geopressure and its superficial effects has long played an important role in petroleum exploration. There is always an element of risk with any drlling operation but the pressence of abnormally pressured sequences can significantly increase these difficulties, successful drilling requires the use of every means of detection at  our disposal. Hydrocarbon exploration and petroleum reservoir characteristics in bengal delta is much more affected by geopressure anomalies and before drilling need analysis the regional geology of prospect area. 

 

Hydrocarbon reservoir does not depend on artifical man made boundary of country but depends on regional geological history. Hence discussing hydrocarbon reservior status in coastal & offshore part of Bangladesh considering subsurface geopressue of regional geology. Bengal basin is part of Myanmar, Bangladesh & India and basinal part of Bangladesh is realtively at down slope of Myanmar and in middle of Bengal basin. Sedimentary thickness of Bengal basin is almost 20-22km thick and in the form of arch & synclinal basin design represent more sedimentary thickness.  

 

Resulting force of plate movement has great effect on sedimentary bodies, twist the sedimentary body into different sub structure like fold, fault, fracture etc. Sedimentary body of southern part of Bengal basin is divided into two types of structural view by thrust fault, i.e. uplifted south eastern fold belt and north western plain land with southern coastal & offshore part.  South eastern foldbelt is highly compressed and  pressurized rock body. High compression makes fault and fracture inside formation thus affect the hydrocarbon reservoir, seal and migration. South eastern uplifted foldbelt & adjoining area is quite unpredictable for high intensity of geopressure anomalies. Hydrocarbon of this area especially natural gas could leach to nearest closure or even can be out through surface feature linked with internal fracture or fault of rock sequences.

Geopressure trend in coastal & shallow sea

From limited scope a study of regional geo pressure is made from nearby offset well data of coastal & shallow sea of Bangladesh. Above figure represents actual status of geo pressure and hydrocarbon reservoir in this area. In Bangladesh hydrocarbon reservoir is mainly in formation of Mio-Pliocene age above identified over pressure zone. It shows Semutang, Halda & Magnama is highly compressed & pressurized and subsurface is greatly affected by fault & fracture and geo pressure is upward with depth. The Indo-Burman range is located to the east of Magnama and passage by Semutang & Halda marks the nearby zone of plate collision. By the Late Eocene time the direction of Indian plate convergence changed from north to northeast with increasing collision with Southeast Asia. Deformation is strongest as plate move north eastwards towards the Indo-Burman range. This is evidenced by the increase in shallowness of over pressure zone.

South eastern fold belt & adjoining coastal area will not be good hydrocarbon reservoir as commencing high convergence, high angle inclination & less extension of sedimentary sequences, affected factor of plate tectonics and very close to subduction zone. Drilling in this area might be less prospective and hazardous. Sangu, Kutubdia, Sandwip, Shahbazpur, Hatia, Sonadia and Char Kajol is comparatively at down slope of coastal area, gently sloping down to south & southwest direction towards offshore and is relatively far away from the Indo-Burman range and less affected by plate tectonics. Most structures are hydrocarbon charged and some of the drilled wells in this area are in production track. Nearby location like CharJabbar, Manpura, Bhola and adjoining structures are highly prospective for hydrocarbon considering regional geo pressure trend and geological criteria. Over pressure of BODC and Bina well location is relatively deeper and almost has no effect of plate tectonics. South western Bengal basin of Bangladesh part can be affected by Barishal-Chandpur gravity high and hydrocarbon reservoir in this area can also be affected. Though don’t have available data of this region but assume offshore will be hydrocarbon rich area.        

Comparatively geo pressure trend is gently sloping down from south eastern coastal region to deep offshore of Bengal basin in the direction of south & southwest. Thick sedimentary sequence with overlying water body of deep offshore area is less affected by plate tectonics and has wide extension of rock formation can be indicative of good hydrocarbon reservoir. Sedimentary rock of Bengal basin is organic rich, hydrocarbon maturation time & migration is also favorable for hydrocarbon generation. As less effect of plate tectonics in sedimentary rock sequence of deep offshore area may have possibilities of oil discovery in Pleistocene formation at shallow depth and Mio-Pliocene formation above over pressure will be highly prospective for hydrocarbon.  

For hydrocarbon exploration seismic data acquisition is primary procedure for defining sedimentary structure of hydrocarbon reservoir. In this regard qualitative data acquisition is most important for proper analysis, facts that work in offshore are quite expensive and hazardous. In view of qualitative data acquisition Multi client seismic survey will not be authentic rather have possibility of data manipulation considering the liability of risky offshore work. Manipulated data will never represent the real subsurface feature other than it may misguide the expensive exploration work. Better option is search for IOC and proceeds for offshore bidding & move on details negotiation for signing PSC (Production Sharing Contract). Seismic data acquisition in offshore under PSC will be more qualitative as cost recovery is related with hydrocarbon discovery of respective offshore block.