TYPES OF SEDIMENTARY BASINS, MECHANISM OF BASIN FORMATION & PETROLEUM HABITAT BY S. K.BISWAS

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1 TYPES OF SEDIMENTARY BASINS, MECHANISM OF BASIN FORMATION & PETROLEUM HABITAT BY S. K.BISWAS

2 BASIN DEFINITION, CHARACTERISTICS & CLASSIFICATION A sedimentary basin is a structurally morphotectonic depression capable of receiving and preserving sediments with a depositional history not withstanding erosion and nondeposition from time to time. A sedimentary basin represents a unit of geological structure, which is filled up with sediments unique to it during a given span of time.

3 A sedimentary basin is characterized by: A distinctive sediment fill Single or multiple depositional cycles Distinctive tectonic framework & basin architecture which define the basin type Single or several phases of tectonic and/or thermogenic subsidence tectonic and/or thermogenic history. One or more tectono-sedimentary episodes defining single or poly history basins Stratigraphic sequences related to tectonic episodes Distinctive geological history indicated by sedimentation cycles

4 The style of evolution of a basin depends on the tectonic realm where it is located It is, therefore, possible to group basins in genetic classes each characterized by its typical structural style, sediment fill and evolutionary history depending on the tectonic regimen to which it belongs Each plate tectonic set up is characterized by genetically related group of basins where structural styles depend on the inter-plate & intra-plate movements typical of the set up in which they occur

5 Mechanics of basin formation and its development, therefore, vary according to the tectonic processes of the set up Thus, basins are classified and grouped under plate tectonic set up and processes involved, viz., Divergent, Convergent & Transform settings respectively characterized by extensional, compressional & transtensional (horizontal stress) movement (Table 1 & Fig.1). Processes responsible for basin subsidence are given in Table 2 & Fig 2.

6 Basins evolve with the plate movement and one type of basin may change into another or abort and remain as a fossil basin depending on the stages of plate evolution. Thus post-sedimentation life span vary (Fig.3). Type of sediment fill, subsidence & depositional histories are important factors for hydro-carbon generation & tectonic history is important for accumulation.

8 MECHANISM OF RIFTING ACTIVE: UPLIFT PRECEEDING RIFTING GRAVITY SPREADING ASTHENOSPHERE DIAPIRISM PASSIVE: SUBSIDENCE IS FIRST EXPRESSION OF RIFTING PURE SHEAR: UNIFORM EXTN. UNIFORM EXTN. WITH SIMPLE SHEAR: INDUCED MANLE CONVECTION NON-UNIFORM EXTN. CONTINUOUS DISCONTINUOUS

9 SUBSIDENCE MECHANISM CRUSTAL THINNING: Extensional stretching along primordial shear zones(figs. 4&5) MANTLE-LITHOSPHERE THICKENING: Cooling of lithosphere following rise of asthenosphere. SEDIMENTARY & VOLCANIC LOADING: Local isostatic compensation of crust and regional lithospheric flexure during sedimentation & volcanism. TECTONIC LOADING: Local isostatic compensation of crust during overthrusting. SUB-CRUSTAL LOADING: Lithospheric flexure during underthrusting of dense lithosphere. ASTHENOSPHERIC FLOW: Dynamic effects of asthenospheric flow due to descent or delamination of subducted lithosphere crustal under-plating. CRUSTAL DENSIFICATION: Increased density of crust due to PT change and/or emplacement of higher density melts into lower density crust.

10 INTRA-CONTINETAL BASINS Sag basins (Fig.11) are formed by sagging of continental crusts due to crustal distension by far field deviatorial stress. Subsidence could be caused by : (i) Increase in crustal density by eclogite phase transformation, (ii) Prebreak up crustal distension by impingement of thermal plume, (iii) Thermal metamorphism of the lower crust, (iv) Mechanical subsidence by isostatic compensation, (v) Thermal subsidence (vi) Tectonic reactivation of older structure, (vii) Changes in intra-plate stress, (viii) Thermal subsidence following intrusion and cooling, and (ix) Subsidence caused by tectonic events at adjacent plate margin.

11 Rift related sag basins may be asymmetric with boundary faults developing Interior Fracture basin. These basins are aborted rifts in continental set up. Sag basins developed by pre-break up distension of continental crust may develop into interior fracture basin before rift failure. Sag basins are favourable habitat for hydrocarbons depending on the environment of deposition, sediment heterogeneity, subsidence and thermal history.

12 RIFTBASINS Intra-continental rifting leading to break up of continent and spreading of ocean generally takes place along the ancient orogenic belts / paleo-sutures (Fig.4) due to crustal extension & thinning as a result of the rise of volcanic plume from upper mantle or uparching of mantle (active rifting depending on the convecting asthenosphere, Fig.5). Two main rifting mechanism: active and passive rifting (Fig 6 & Table 3). The struactural style depends on mode of rift propagation by stress adjustment through transfer zones (Fig.7). Pericratonic rifts are located at the contnental margins or as shelfal horst-graben complex, after successful rifting of the continent and spreading of proto-oceanic trough (Fig. 8 A-C).

13 Intra-cratonic rifts develop within continental crust forming rift valleys, e.g., Gondwana basins. Failed rifts are aborted arm of rrr-triple junction after successful rifting along other two arms leading to separation of continents and creation of proto-oceanic troughs (Fig.8C). Aulacogens are former failed rifts (Fig.9) at high angles to continental margins which have been reactivated during convergent tectonics, so that they are at high angles or orhtogonal to the orogenic belts. Miogeoclinal prisms include proximal ocean floor, continental slopes & rises continental embankments (Fig. 8 D-F). Basins adjacent to the emerging mid-oceanic ridges are Active Ocean basins. In fossil rifts aborted after initial spreading of the oceanic crust, such basins are called Dormant Ocean basins.

14 PETROLEUM HABITAT OF RIFT BASINS Rift basins are ideal habitat for hydrocarbon occurrence due to presence of all the favorable factors required for generation & accumulation: Optimum Thermal History Favorable Tectono-sedimentary History Adequate subsidence / sediment accommodation Heterogeneous sediment fill & organic content Restricted environment Favorable timing of structure formation vis-à-vis migration

15 FORELAND BASINS Formed in collisional dynamic loading set up by continentcontinent, arc-arc and arc-continent collision. A peripheral foreland basin is formed as the elastic lithosphere of the approaching continent flexes under the frontal thrust belt of the over-riding continent (Fig. 12). Retro-arc foreland basins are formed by arc-continent collision and occur on the continent side of compressional arc formed during subduction of oceanic plate (Fig.13). Intermontane foreland basins are formed by basement cored uplifts in foreland settings (Fig.14) Inter-orogenic foreland basins occur between two orogenic belts involving collision of three continents. When two continents collide on either sides of a third continent overriding it, the common peripheral foreland basin between the two-obducting continents becomes this type of basin.

16 Foreland Basins - Petroleum habitat Peripheral foreland basins with thick wedge of marine sediments are favorable habitat for hydrocarbons. Many large oil/gas fields are located in this type of basin e.g. USA, Canada, Iraq, Iran, Pakistan, & India (Upper Assam basin). The petroleum accumulations in passive margin basins (pericratonic rifts or sag basins) of the partially subducted continent may escape from under thrusting or may be preserved under brow zone of thrust belt of obducting continent. Organic rich sediments of paleo-passive margin are carried down in the subduction zone into a favorable thermal regime where oil/gas is generated and migrate up into the peripheral foreland where it is trapped.

17 ARC-TRENCH BASINS (Fig.16) Fore-arc trenches occur at the threshold of the oceanarc subduction zone (Fig.17). The subsidence mechanism is controlled by negative buoyancy of the subducting oceanic lithosphere by slab-pull forces and by isostatic tectonic load of the accretionary prism over the subduction zone (Figs. 18 & 19). Intra-arc basins (Fig.20) are located on arc platform between volcanic fronts. Subsidence caused by relative plate motion, plate-boundary forces, asthenospheric flow, regional isostasy, magmatic withdrawal or by gravitational forces.

18 Back arc basins (Figs. 20 & 21) occur behind continent margin arc and behind intra-oceanic magmatic arcs. Produced by rifting between arc massifs during sea floor spreading. Back arc basin between Remnant and Active arcs are called Inter arc basin (Fig. 22). Remnant Ocean basins are shrinking basins caught up between obliquely colliding continents and/or arctrench systems, ultimately subducting or deformed between suture belts (Fig. 23). Superposed basins/piggy back basins (Fig. 24) are formed on the thrust belts by sagging due to interaction of thrust sheets and carried by moving thrusts.

19 PETROLEUM HABITAT OF ARC-TRENCH BASINS Arc-trench basins with thick pile of deep water organic rich sediments lack thermal maturation. However, if the sediments are carried down by subduction into favorable thermal regime, hydrocarbons could be generated and migrate up into favorable traps in residual fore-arc basin or in structures of accretionary prism as seen in west coast of N. America, Alpine thrust belt of Italy and Tripura fold belt of India. In this set up, Back arc basins are most favorable petroleum habitat as the basins are similar to rift basins involving extensional stretching, mantle arching and accompanying volcanic activity.

20 TRANSTENSIONAL BASINS (Fig. 25) These basins develop by lithosphere shearing resulting in dislocation or pulling apart of the lithosphere and hence called pull-apart basins. Such basins occur along trends of transform systems where ever en echelon fault segments, curving faults, or branching faults are arranged in a releasing orientation with respect to the direction of plate motion. Such basins occur between parallel or overlapping strike-slip faults or in their releasing bends, fraying ends or between the main and its conjugate fault systems (Fig. 26 A-B).

21 TRANSPRESSIONAL BASINS (Fig. 26D) These basins are formed by down warping of lithosphere in constraining orientation with respect to plate motion in transform segments. These basins occur with convergent strike-slip faults along restraining bends with thrust margins that results in flexural subsidence due to tectonic loading. TRANS-ROTATIONAL BASINS (Fig. 26C) These fault bound basins develop between blocks that rotate differentially in a shear zone around a sub-vertical axis in the same direction of the principal shear stress. Rotation is clockwise in right simple shear and anticlockwise in the case of left simple shear. Block rotation on sub-vertical/vertical axis yields prominent triangular or rhomb shaped basins that may be extensive in zones of distributed shear.

22 PETROLEUM HABITAT STRIKE-SLIP BASINS Transtensional Pull-apart basins have the same properties as the rift basins and provide most favourable conditions for oil gas generation and entrapment with adequate structuring, basin subsidence and complex depositional environment. The Transpressional & Rotational basins do not favor accumulation due to complex structuring. In most cases post-dates migration as formed during rift inversion.

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