Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021 Developments in the structural geology of rifts over the last decade and their impact on hydrocarbon exploration C. K. MORLEY Department of Petroleum Geology, University of Brunei, Gadong 3186, Brunei Darussalam Abstract: Three different aspects of rift geometry and their impact on hydrocarbon exploration are examined: rift architecture, fault angle and basin inversion. Rift architecture is controlled by fault geometry, which itself is heavily influenced by pre-existing fabrics. At the largest scale, rifts are divided into segments tens to hundreds of kilometres long. They can be joined along offset segments, separated by rift jumps and gaps, or terminated in splays. Rift jumps and gaps provide the entry points for large rivers to enter rifts and create along-axis drainage systems, while rift offsets are areas where anomalous structural patterns develop in response to oblique extension. Within rift segments transfer zones between boundary faults mark important changes in rift geometry and are the preferred sites of coarse clastic sedimentation. In the Gulf of Suez, for example, the structures and the syn-rift reservoir units within two major transfer zones are associated with over four billion barrels of reserves out of a total of six billion barrels. The way individual tilted fault blocks terminate include strike ramps, forced folds, rhomb-blocks, transfer faults, and major and minor cross-strike faults. In areas of relatively poor seismic data quality the choice of fault block termination interpreted on geophysical maps can make a considerable difference to structural interpretation. There has been considerable debate as to whether rifts are composed of high-angle (45-75 ~ faults or a mixture of high- and low-angle faults. Low- angle faults occur in the sedimentary sequence of rifts, where they occur in a variety of structural settings. Such faults can have a significant economic impact in field development. The presence of low-angled basement faults is more problematic. They do apparently occur in rifts, both due to reactivation of older faults and as completely new faults. Rifts can evolve through a variety of low- and high-angle fault structures with time, which impacts trap creation and destruction. Rifts commonly undergo changes in their stress regime which may lead to basin inversion features. This affects trap development and destruction and can lead to a variable subsidence history for basins, as the basin bounding faults change their senses of motion. The recognition of inversion features in many rifts indicates that this is a very important aspect of rift history. Prior to the late 1970s a relatively small number 1976; Rehrig & Reynolds 1980; Davis & of workers studied the structural geology of Hardy 1981; Wernicke 1981, 1985). continental rifts; these include Gregory (1921), 2. Definition of rift architecture by seismic Quennell (1956), Robson (1971), Baker et al. reflection profiles (e.g. Rosendahl et al. (1972), McConnell (1972), Illies (1974) and 1986; Cheadle et al. 1987; Ebinger et al. Garfunkel & Bartov (1977). Much of this early 1987). work focused on the surface geology and 3. Recognition of complex evolution of geomorphology in the Rhine Graben, East faulting in rifts beyond the simple models African rift system, Dead Sea Rift and the Gulf of domino faulting. of Suez. During the 1980s a considerable 4. Geometry of basin inversions. amount of new data, especially seismic reflec- 5. Definition of the influence of syn-rift tion data, added significantly to the under- structure on sedimentation. standing of extension ,1 provinces, although 6. Detailing of fault geometries in rifts. many of the concepts about rift structure that developed during that time existed previously in This paper concentrates on three main themes: some form. Some of the main advances and rift architecture, fault angle and basin inversion controversies that arose during the 1980s are and their impact on hydrocarbon exploration. listed below. This is a potentially large topic but it does help limit which important advances are examined, 1. Recognition of low-angle faulting in base- for example, topics dealing with rift mechanics ment rocks of the Basin-and-Range Pro- and volcanism are largely ignored. Salt tectonics vince (Wright & Troxel 1973; McDonald are also very important in some rifts, particu- From Lambiase, J. J. (ed.), 1995, Hydrocarbon Habitat in Rift Basins, Geological Society Special Publication No. 80, pp. 1-32 Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021 2 C.K. MORLEY SW NE Kilometres I 0 12 --3 --4 15 --6 --7 --8 --9 --10 +.r _11 --12 V=H Fig. 1. Cross-section accoss the SE Rukwa rift (see Fig. 4 for location), based on seismic line TVZ-2 (Morley et al. 1992a). Lake beds are of Upper Miocene-Pleistocene age and Karroo is dominantly Permo-Triassic continental deposits. The following points about half-graben geometry are illustrated by the figure: (1) the major boundary fault (Lupa fault) controls the overall sediment wedge geometry; (2) reactivation of the boundary fault (Karroo and Tertiary times); (3) minor fault, particularly antithetic faults, are best developed near the flexural margin. larly in the North Sea. However, this is not a Schlische & Olsen 1990). The two main basin fundamental structural style to rifts alone and in geometries controlled by boundary faults are the interest of space is ignored in this paper. half-grabens and full-grabens. Some of the basic relationships between minor fault geometry and Rift architecure major fault geometry that are commonly found in these basins are listed in Table 1. Before the 1980s the basic elements of large-scale Examination of recent earthquakes is impor- rift structure had been established. Half-graben tant for understanding how the individual and full-graben geometries were known both displacement increments are built up on faults from modelling and from natural examples (e.g. (Fig. 5). For large earthquakes (magnitude 6.5 Cloos 1936; Dunbar 1949), and changes in or more on the Richter scale) fault strike lengths structural style at discrete zones (hinge zones, of tens of kilometres may be activated, the active accommodation zones, transfer zones) had also surface fault trace commonly forms a series of been noted (Moustafa 1976). Variations in rift en-echelon segments (e.g. Stein & Barrientos segment geometry (offsets, jumps, gaps, splays; 1985; King et al. 1988). The instantaneous or see Nelson et al. 1992 for a summary) had been coseismic deformation associated with such examined in some detail (e.g. Baker et al. 1972; earthquakes produces absolute (not relative) Illies 1974). However, a detailed knowledge of uplift of the footwall and downdropping of the the sub-surface configuration was lacking. These hanging wall in the order of tens of centimetres. geometries are discussed in this section. The magnitude of the hanging wall displacement The investigations of the 1980s, in particular is 6-10-times larger than the footwall uplift those of Project PROBE in East Africa, showed, (Stein & Barrientos 1985). The coseismic (elastic) through high quality seismic reflection data, the deformation introduces an isostatic imbalance details of the changes in geometry (e.g. Rosen- since lower density sediments have replaced a dahl et al. 1986; Ebinger et al. 1987; Dunkleman similar volume of basement rock in the hanging et al. 1988). Some boundary faults were large, wall (Fig. 5). Erosion of the footwall uplift also displaying up to 10km of extension and up to contributes to the isostatic imbalance. The 7 km of syn-rift fill in the hanging wall (Figs 1-3, isostatic (post-seismic) response takes the form and Fig. 4 for location). Such large faults exert of a gentle vertical uplift across the fault zone a first-order control on extension, subsidence, and occurs on the time scale of tens to hundreds basin geometry and sedimentation patterns in of thousands of years. This uplift, which rifts (Rosendahl et al. 1986; Morley 1989; decreases in magnitude away from the fault Downloaded from http://sp.lyellcollection.org/ by guest on September 29, 2021 STRUCTURAL GEOLOGY OF RIFTS 3 East-side Central Tanganyika 0 150 km 0 3 --., 4 5 6 Top Precambrian basement km Lokichar Basin S N 0 50 km 100 km 0 -. ........ i i .0t Along Strike Change in ~. 2 ; Fault Activity with Time 4 "--- \ r km b Horizon within Horizon Lower Miocene / within Upper Middle Miocene Miocene Lake Rukwa, Lupa Fault NW 50 km 100 km 150 km SE o i i i o 1 1 2 Base Upper 2 3 Miocene Sequence 3 4 4 5 t--"-6 6 ""7 7 8 Base ,,~ ~ ~"""" 8 9 ,, Karroo . " "'" 9 10 10km T T ""''- ......... '''"" Along Strike Anticlines Showing Smooth Increases and Decreases in Boundary Fault Displacement Fig. 2. Examples of major fault geometries from rifts illustrating the displacement patterns of the faults by plotting changes in throw, derived from seismic data, along the strike of the fault (see Fig. 4 for location). The data illustrate a simple case (a) and complex cases where faults are either reactivated (c) or change timing of activity along-strike (b). (a) Lake Tanganyika, east-side boundary fault, illustrating a simple decrease in displacement from a maximum at the centre. (b) Lokichar Basin, Kenya Rift, demonstrating abrupt younging of the main fault activity to the north (this may reflect linkage of younger and older faults). (c) Lake Rukwa, Lupa Fault, displaying skewed displacement maxima, similar displacement trends for both the Karroo and Tertiary age rifting and along-strike anticlines as displacement varies along the fault. Displacement variations may reflect linkage of three separate large faults (each one corresponding to a displacement maxima). zone, enhances the footwall uplift, reduces large normal faults that penetrate the upper hanging wall subsidence and gently warps the crust are the result of multiple earthquakes plus fault plane.