Recent hydrocarbon plays are actively carried out in the following structural settings: (1) Rift basins formed by planar or listric normal faults; (2) fault-propagation folds, fault-bend folds and detachment folds in thin-skinned tectonics as well as basement-involved fold-and-thrust belts; (3) triangle zones and core wedges of subthrusts; (4) zones of inversion tectonics; (5) pull-apart basins of strike-slip tectonics; and (6) subsalt and suprasalt structures formed by salt intrusion or overburden pressure. The integration of results of such plays has given a better understanding of tectonic processes, timing, and structural styles of petroleum traps, which can enhance the theoretical capability of various structural geology tools such as the balanced cross-sections, as well as numerical and physical models. A review of these developments is presented here with emphasis on the implications and problems of the structure-formation models for trap interpretation and petroleum exploration. Although the current computer models for analysis and prediction of sub-seismic faults are far from actuality, the utilization of boundary element method or particle flow code in structure-formation models derived from geologic studies leads to more realistic computer tools for trap analysis and seismic interpretation in hydrocarbon plays.
Petroleum exploration requires an exact cross section. Balanced cross section construction (Dahlstrom, 1969) is a very useful way to extrapolate surface structures to the depths. The basic concept of his method is that the geometric features of any constructed section must be retrodeformable to the restored section. Bed lengths and areas must be equal both in deformed cross section and undeformed restored section. Balanced cross section construction has been very successful in understanding complex structures of the fold and thrust belts of the orogenic belts, especially for duplex structures. But an application of the method to the accretionary terranes is very difficult, because original successions are not clarified. However in such a case, the concept of balanced cross section is essential to understand thrust structures. Some examples of the duplex structures of the Shimanto Terrane in Kyushu, Japan are described.Key words: balanced cross section, thrust, duplex, fault-bend fold; normal fault, Shimanto Terrane, Kyushu
Deformation of thrust structure is generally reconstructed by using the geometrical balanced cross section method. Ramp angle, its length, volume of hangingwall displacement on the detachment and thickness of sediments deformed on the ramp control the analyzed fold structures. These geological elements are directly influenced by a state of subsurface stress and rock properties. However it is difficult to include the influence of rock properties in the section which is constructed in geometrical balancing. In this paper, two geological sections in the Niigata basin are reconstructed by the Virtual Basement Displacement (VBD) method (Kodama et al., 1985), which simulates deformation and fracturing of the strata in a sedimentary basin. The distribution and magnitude of the strain concentration and fractures in these sections were analyzed quantitatively. The result of the simulation suggests that the strain is concentrated around volcanic bodies and reactivation of basement fractures in the well MITI Niigata-heiya section A-A', and some anticlinal anomalies had existed in deeper part of synclinal area in the section B-B' during the structural deformation history. The folds are formed with vertical displacement and the shortening volume of the folds seems to be less than the result of the balanced cross section of the geometrical method. We can comprehend some tips, images and ways of thinking during the numerical analyses considering dynamic picture and kinematic picture. The understanding of rock properties and subsurface stress will lead us to effective exploration, especially to assess the thrust structures.
The eastern margin of the Japan Sea has rifted in the early Miocene. Then it hascompressed during the late Pliocene through the Quaternary. This change of stress caused basininversion along the margin, but the inversion structures vary mainly due to difference of therifting structures. In this paper, three types of basin inversion were defined. Type A basininversion is characterized by simple inversion of half grabens. Miocene normal faults reactivatedas reverse faults and the whole grabens have been uplifted. Many basin inversions of this typehave been widely formed in the Sado Ridge and Mogami Trough in the southern part of theeastern margin of the Japan Sea, to the west of the Tohoku Arc. Type B basin inversion isdefined by anticlinoriums in the major rift basins along the coastal zone of the Tohoku arc. Therifts have been partly uplifted by reverse faults and asymmetric anticlines, suggesting that partsof the extensional faults in the rifts have reactivated. Type C basin inversion forms the OkushiriRidge which continues in the N-S direction along the eastern margin of the Japan Basin to thewest of Hokkaido. It is not clear that the ridge has overprinted on the preexisting extensionalfaults. Reverse faults in the three types of basin inversion are accompanied by asymmetricanticlines or anticlinoriums. The profiles of the anticlines suggest that hinges of the anticlineshave been fixed during the growth of the anticlines. These structure and growth pattern of theanticlines can be explained by listric reverse faults and detachments at 10-20km in depth. Thegeologic structure related with extensional tectonics are generally obscure due to the lack ofpre-rift sedimentary sequences and extensive activity of volcanism in the rifting stage.Consequentry, it is difficult to clarify the relationship of the major fault systems between theextensional and compressional stages.
Late Cenozoic tectonic history around the southern part of the Japan Sea and Kyushu is reconstructed on the basis of offshore reflection seismic survey and borehole stratigraphy together with geologic information on land. Three tectonic episodes are identified in the study area as follows; (1) Paleomagnetic data suggest that western half of the Japan Sea was generated through Early to Middle Miocene rifting and clockwise rotation of southwest Japan. Seismic profiles around the southern margin of the Japan Sea show extensional grabens filled with late Early Miocene sediments. Coeval deformation in the Fukue Basin off northwestern Kyushu, which is located around rotation pivot of the rifted sliver of southwest Japan, is characterized by right-lateral leaky wrenching along a NNE-SSW structural trend. (2) Southern margin of the Japan Sea suffered extensive inversion around the end of Miocene, probably caused by resumption of subduction of the Philippine Sea Plate. ENE-WSW trending folds (San'in Folded Zone) converge into Tsushima Island at the western end of southwest Japan. Shortening of the southern part of the Japan Sea was accommodated by left-lateral movement along the Tsushima-Goto Fault on the western margin of the back-arc basin. In a sharp contrast, Goto-nada and Amakusa-nada Seas, and central Kyushu began to subside with many extensional features in the same period, which may be linked to mantle upwelling. (3) In Quaternary, study area is divided into three tectonic domains: Inner Zone of southwest Japan is characterized by simple shear deformation caused by right-lateral movements on the Median Tectonic Line (MTL). Southern part of the Japan Sea is undulated under E-W compressive stress. Central Kyushu continued to subside alternatively controlled by pull-apart basin formation on the MTL and mantle upwelling.
The Yufutsu oil and gas field, Tomakomai City, southern Ishikari Plain, centralHokkaido, has its naturally fractured reservoir in Cretaceous granitoids and overlying Eoceneconglomerates, which form a large horst-complex delineated by normal faults. Stratigraphic andstructural features of the Yufutsu field and its surrounding areas indicate that prominentextensional deformation shaped the horst during the late Oligocene to early Miocene, when the lateCenozoic sedimentary basin in the southern Ishikari Plain originated and subsequently developedstepwise from an extensional-transtensional (?), relatively localized basin to a transpressionalforeland basin of a regional scale. Understanding of these Cenozoic tectonic processes leads to anexploration concept which counts basement horsts for traps with fractured reservoir and deeplyburied coal-bearing strata for hydrocarbon sources. The MITI Umaoi well, drilled 25km north ofthe Yufutsu field, proved this concept practical even in a subthrust setting, as it hit a wellpermeable fractured reservoir within a basement horst. A new lithostratigraphic unit, Minami-naganuma Formation (Upper Oligocene), is formally proposed.
Oil Fields in the offshore Zaire (D. R. Congo) area are located in the Congo basin. This basin was developed as a result of South Atlantic rift that was initiated towards the end of the Jurassic period. The sedimentary section in this area is divided by the Aptian salt into a presalt nonmarine sequence and a postsalt marine sequence. The structural style of the Pinda formation (postsalt) is characterized by salt movement and associated with growth faults. Salt movement at the early stage of the Pinda deposition developed many down-to-basin normal faults. While significant salt diapirs raised and Lower Pinda formation slipped toward the basin and leaned eastward Upper Pinda carbonate deposited. The salt movement and stratigraphic expansion were terminated during the deposition of Kinkashi formation (Early Cenomanian). As a result of this salt tectonics, the Lower Pinda formation was separated into several fault blocks by down-to-basin listric faults. Those fault blocks are similar to “rafts” that were recognized in the Cuanza Basin (Duval and Cramez, 1992; Jackson and Talbot, 1991; Jackson, 1997). The stratigraphic expansion is calculated approximately as 1 to 1.3km/million years. However, at the early stage of the Upper Pinda carbonate deposition, significant salt deformation occurred, which is calculated to be on the order of approximately 3km per million years. Carbonate and sandstone rocks of the Pinda formation serve as the main reservoir. The salt diapirs controlled the distribution of sandstone and the reservoir quality of carbonates.
An ambitious structural interpretation based on balanced cross section methodresulted in the first exploratory well targeted the footwall anticline structure in the Papuan FoldBelt where no seismic was available. In the past, when drilling proved that a hanging wall anticline was shallowly cut by a thrustand did not involve reservoir horizons, the well was usually abandoned due to the structuraluncertainty of the footwall. This time, however, interpretation of the sub-thrust was carried outfor a footwall structure whose hanging wall was shallowly cut by a thrust. An anticline structureof the footwall was presumed based on constraints of the morphology of the hanging wall, dip dataof the footwall, displacement of the thrust and line-length balancing method. This presumptionanticline was confirmed by drilling. The SCAT (Statistical Curvature Analysis Technique; Bengton, 1981) analysis applied to thefootwall structure suggested that the well is located on the west-northwest plunge of the anticlineand that the structure is likely to be an antiformal stack. The well data and its structuralinterpretation confirm that structures in the Papuan Fold Belt do not always fit a simple ramp foldmodel. Newly developed and published structural models in the Papuan Fold Belt are also introducedand discussed in this paper.
The West Natuna Sea Basins are the Tertiary, inverted, intra-continental rift-basins in the Sunda Shelf, Northern Indonesia. The basins include remarkable oil producers as well as most other Indonesian Tertiary basins. Inversion-anticlinal traps in the basins have been classic examples of a positive contribution of inversion tectonics to formation of petroleum systems and plays. The Tertiary mega-sequence of the basin includes a Middle to Upper Eocene lacustrine deposit, an Oligocene fluvial to deltaic deposit, an Early Miocene muddy facies deposit, a Middle Miocene deposit of sand-dominant deposit, and a Late Miocene through Recent alternative mud-sand deposit. The West Natuna Sea stratigraphy remarkably lacks the thick Early to Middle Miocene carbonates that are normally well-developed in the most Indonesian Tertiary basins. The Oligocene and Lower Miocene sandstones are productive reservoirs, and widely deposited Oligocene and Lower Miocene mudstones provide regional and top seals, for petroleum systems in the basins. Four petroleum systems are identified in the southern West Natuna Sea based on the magnitudes of inversion tectonics and resulting thermal “kitchen” that developed. Systems (1) and (2) are based on petroleum-charging kitchens, such as the Malay Basin or the Bawal Graben. These kitchens are distinguished from each other by carbon stable isotope ratios of alkanes and aromatics of crude oils and by a biomarker. Systems A and B are by tectonic styles related to petroleum accumulations, such as basement highs or inversion anticlines. The four petroleum system are recognized from west to east and north to south in the area; such as System (1) A (Belida oil field), System (1) B (Tembang, Buntal and Bintang Laut gas pools), System (2) B (Forel oil pool, Belanak oil and gas field) and System (2) A (Udang oil field). System (1) forms much larger petroleum-accumulations than System (2). Sizes of the accumulations are clearly related to the sizes of kitchens that charged the accumulations.
The collision of the Kuril arc against the Northeast Japan arc has made a conspicuous crustal structure from the Hidaka mountain range to the Ishikari-Tomakomai lowland, Hokkaido, Japan, since Miocene. Recent advance of deep seismic reflection studies has revealed that the Kuril arc lithosphere is delaminated at about 23km deep in the lower crust in the Hidaka collision zone. The upper half of the lithosphere (upper crust+upper portion of the lower crust) is thrust westward on the Northeast Japan arc, whereas the lower half (lower portion of the lower crust+upper mantle) descends down. The wedge of the Northeast Japan arc lithosphere intrudes eastward into the delaminated Kuril arc lithosphere. The structure is called as a “delamination-wedge structure”. In the western foreland area of the Hidaka mountain range, the west-verging fold-and-thrust belt occurs more than 70km wide involving the pre-Tertiary strata. The activity of the belt has shifted westward since the initiation of the collision. The shortening length in the foreland fold-and-thrust belt is about 60km, which is nearly equal to the delamination-wedge length toward the colliding direction. The two lines of evidence mentioned above, the westward shift of the activity and the coincidence of both lengths, indicate that the fold-and-thrust belt has been growing associated directly with the formation of the delamination-wedge structure.