John D. Pigott

Episodic Rifting and Subsidence in the South China Sea

The South China Sea experienced at least three stages of rifting and two intervening stages of sea-floor spreading since the Early Cretaceous. Its evolution can be described by an episodic model of tectonism, one of thermal cooling and subsidence, pulsed by temporally and spatially confined heating events. Analysis of regional geologic and geophysical data suggests episodes of rifting and associated thermal activities initiated during the Late Cretaceous, the late Eocene, and the late early Miocene. The rift system corresponding to the first episode trends northeast-southwest, whereas those of the second and third trend east-west. These two trends coincide with the orientations of the major tectonic lineations within the basin. Age estimates from heat-flow and bathymetric data suggest the oceanic crust in the Southwest subbasin is considerably older (55 Ma) than that in the Northwest (35-36 Ma) or East (32 Ma) subbasins. The episodic tectonic model is supported by basin subsidence analysis. Subsidence, especially subsidence rate curves derived from the regional well data, demonstrates that unlike a classic Atlantic passive margin, thermal subsidence in the South China Sea was punctuated by rapid subsidence events which are chronologically consistent with the rifting episodes. In terms of hydrocarbon potential, the episodes of rifting and drifting would be conducive to the development of overprinted structures and the deposition of several discrete transgressive packages of source rocks and reservoirs, separated by widespread unconformities. The thermal maturity of sedimentary organic matter affected by episodic rifting and subsidence may be greater than expected on a purely passive margin of equivalent age that had not experienced repeated heating.

Tectonic controls of Phanerozoic sedimentary rock cycling

The cyclic nature of the Phanerozoic sedimentary rock distribution, carbon-sulphur coupling, and material transfer among sedimentary reservoirs appears to be controlled by tectonic factors. The distribution of preserved sedimentary mass in terms of rock mass remaining U. geologic age shows a minimum c. 300-350 Ma ago, which separates two subcycles of erosion and deposition of sedimentary rocks. The older subcycle was interrupted because of the major continental collisions of the Devonian and late Carboniferous. These collisions resulted in a reduction of outcrop areas of rocks of the older cycle relative to their masses, leading to a decline in the probability of destruction and an increase in half-life of these older sediments. A strong correlation exists between the long-term cyclicity in the Phanerozoic global sea level curve and the distribution of carbon and sulphur among their major exogenic reservoirs. This correlation is related to two principal tectonic modes of the Phanerozoic: oscillatory and submergent. It is postulated that the submergent mode of active plate convergence, obduction and subduction of sediments, large ridge volume, and high sea level gave rise to low erosion and sedimentation rates, less restricted environments of carbonate deposition, and relatively high atmospheric CO2 levels (high temperatures ?), resulting from an increased rate of production of CO2 from diagenetic and metamorphic reactions at subduction zones. As sea level rose carbon was transferred from the sedimentary reservoir of reduced organic carbon to that of oxidized inorganic carbon in limestones, whereas sulphur moved from the oxidized sulphate reservoir to the reduced sulphide reservoir. As sea level fell, reservoir transfers were opposite to those above culminating in the oscillatory mode of generally elevated continental interiors. These reservoir transfers are consistent with secular changes observed in the distribution of δ13C and δ34S in Sedimentary materials during the Phanerozoic. Petrographic examination of Phanerozoic oolite formations shows that öoids with preserved calcitic relict textures are characteristic of pre-Carboniferous carbonate rocks, whereas öoids with relict textures indicative of initial aragonite mineralogy are dominant in rocks of younger age. These changes in öoid mineralogy may be interpreted as reflecting changes in CO2 levels of the ocean-atmosphere system consistent with the above tectonic considerations. Atmospheric CO2 levels were higher prior to Carboniferous time, favouring formation of calcitic öoids and skeletal parts; after the Carboniferous, CO2 levels fell and aragonite and Mg-calcites of greater than 8 mol % Mg increased in abundance as precipitates.

Basin superposition on the northern margin of the South China Sea

Abstract Basin superposition on the northern margin of the South China Sea is a product of pulsed basin development with geometries which changed through space and time, of changing strain fields, and of tectonically inherited, pre-existing zones of crustal weakness. This region of the South China Sea was welded together or assembled prior to the Cretaceous. Three distinct episodes of continental disassembly and accompanying basin evolution ensued, each episode commencing with rifting and the formation of grabens or half-grabens and each terminating with a gentle subsidence phase. The major trends of the faults and basins varied from predominantly NE for the first episode, through NE-EW for the second, to EW-NW for the third. The first episode occurred during the Late Cretaceous to Paleocene as a back-arc extensional domain induced by the dip steepening of a subducted oceanic slab. The second basin episode of the late Eocene to early Oligocene was initiated by the collision of the Indian plate with the Eurasian plate. The continued penetration of the former into the latter led to the tectonic escape and subsequent clockwise rotation of the Indochina block, creating regional transtension and left-lateral motion along the Red River Fault. The third episode of basin development occurred during the middle Miocene as a reactivated product of this collision. Owing to the clockwise rotation of Indochina, the approximately E-W trend of the simple shear principal displacement zone changed to northwest. By the Pliocene, the eastward extrusion of South China had exceeded that of Indochina, reversing the sense of Red River Fault displacement to right-lateral. A clockwise rotation of the Red River geosuture, together with considerations of the influence of zones of inherited crustal weakness, provides a model for the observed episodic changes in regional strain. Such a model best explains the post-Cretaceous evolution of superimposed basins on the northern margin of the South China Sea.

Interstitial water chemistry of Jamaican reef sediment: sulfate reduction and submarine cementation

Abstract Chemical and isotopic analyses of pore waters from Jamaican reef sediment suggest the importance of microbial sulfate reduction as a major control upon the origin, distribution, and composition of submarine cements in this fringing reef setting. Fore-reef sediment pore waters exhibit active sulfate reduction and enrichment in 18 O which is consistent with associated active magnesian calcite cementation, alkalinity consumption, and cement enrichment in 18 O. Conversely, lack of widespread submarine cementation in the back-reef corresponds to the diminished resupply of sulfate coupled with input of CO 2 -charged meteoric water from a nearby unconfined aquifer into the more stagnant pore waters which lower pore-water magnesian calcite saturation states and preclude active submarine cementation.

Aspects of sedimentary basin evolution assessed through tectonic subsidence analysis. Example: northern Gulf of Thailand

Abstract Tectonic subsidence and subsidence rate analyses were conducted using a forward burial technique for the Cenozoic sediments of the northern Gulf of Thailand, a region presently bounded and intersected by major strike-slip fault systems. Basins represented by the seven wells studied are the Thon Buri, Hua Hin, Chumphon, Kra, and Pattani basins. The total observed subsidence was stratigraphically calibrated using well biostratigraphy and/or regional seismic stratigraphy. Tectonic subsidence was subsequently determined assuming local Airy isostasy by correcting decompacted sediments for sediment loading and variations in paleowater depths. Statistical comparison of the observed tectonic subsidence profile versus the theoretical thermal subsidence profile reveals zero-intercept times of incipient thermal-rifting and furthermore helps differentiate times of thermal subsidence from episodes of fault-controlled mechanical subsidence. Differences in tectonic subsidence, tectonic subsidence rates, and in the zero-intercept times of thermal rifting imply the Paleogene thermal associated rifting of the northern Gulf of Thailand was neither restricted spatially nor universally synchronous among the basins, but instead both spatially and time transgressive. Although coupled thermal-mechanical subsidence played a major role in the evolution for most of these basins, in some basins, e.g. the Thon Buri and northern Kra basins, subsidence was not thermally initiated. Instead, these basins experienced relatively slow-paced sediment loaded subsidence until a sudden fault-associated acceleration commenced in the Pliocene. Additional evidence for temporal and spatial changes in local strain is demonstrated by observed asynchronous episodes of “see-saw” subsidence-uplift of the basin floors. For example, while the northern Hua Hin Basin experienced Miocene-Pliocene alternations of subsidence and uplift, portions of the Pattani Basin to the southeast underwent periods of subsidence and uplift which differed in magnitude and phase. In comparison to the tectonic subsidence rates compiled for basin classes worldwide, these Gulf of Thailand basins exhibit time-transient characteristics of cratonic, rift, and wrench style basins. The opening of the northern Gulf of Thailand, basin formation, and subsequent basin evolution have varied both in time and space. Therefore, in terms of hydrocarbon potential, it would be both inappropriate and inaccurate to suggest the thermal histories and subsequent hydrocarbon maturities of these wrench-associated basins to be equivalent. Though tectonically related, each basin has had its own geohistory.

Post-Jurassic tectonic evolution of Papua New Guinea

Abstract Synthesis of available geologic and geophysical data for Papua New Guinea yields a series of kinematically constrained tectonic reconstructions. These reconstructions document the post-Jurassic evolution of the northern margin of the Australian plate from a rifted, passive continental margin to one composed of accreted, tectonostratigraphic terranes undergoing sinistral oblique transpression. The allochthonous terranes represent marginal basins and fringing island arcs which began docking to the Australian plate in the Miocene. At present, assemblage forms a diffuse suture zone between the Australian and Pacific plates, structurally responding in a complex fashion to the regional oblique compression.

Analysis of petroleum system criticals of the Matruh–Shushan Basin, Western Desert, Egypt

A systematic analysis of petroleum system criticals can provide a robust review of a basins hydrocarbon potential through time and space. The ten essential petroleum system criticals that express the extensive and intensive variables are: source generation volume (Sgv), source-rock richness (Sgr), source-rock quality (Srq), source-rock maturity (Srm), reservoir rock volume (Rrv), reservoir rock quality (Rrq), reservoir rock hydrocarbon type (Rrhct), reservoir rock seal and closure (Rrsc), flux migration path (Fmp) and petroleum system timing (PSt). The Matruh–Shushan Basin of the Western Desert, Egypt, forms the basis for an example of the application of this technique. Modelling and empirical data of source-rock criticals reveal that the Mesozoic source generation megasequence is restricted in the Matruh–Shushan Basin. Presently, these areas lie buried at their maximum experienced temperatures. Potential reservoirs in portions of the north and central Western Desert were dependent upon lateral migration path criticals for their charge. Progressive uplift and basin inversion since the middle Palaeozoic provided favourable conditions for lateral migration in the Mesozoic. The main potential source rocks in the present basins are the Lower Cretaceous Alam El Bueib (AEB) and the Jurassic Khatatba. Although both share mixed kerogen types (II/III), they attained their highest levels of thermal maturity at different times. Basin modelling suggests the Lower Cretaceous AEB entered the oil window in the Late Cretaceous, while the Jurassic Khatatba of the deeper part of the basin entered the oil window in the Turonian. Charge risks increase in the deeper basin megasequences in which migration hydrocarbons must traverse the basin updip. The migration pathways were principally lateral ramps and faults which enabled migration into the shallower post-Late Cretaceous structured reservoirs. Basin modelling incorporating an analysis of the petroleum system criticals has outlined the spatial and temporal extent of the different petroleum systems in the Matruh–Shushan Basin and can help guide the next exploration phase. While oil exploration is now focused appropriately along Late Cretaceous and Tertiary migration paths, these results suggest deeper sections may have reservoirs charged with significant unrealized gas potential.

Sulfur and oxygen isotopic compositions of dissolved sulfate in the Orca Basin: Implications for origin of the high-salinity brine and oxidation of sulfides at the brine-seawater interface

Abstract Analysis of sulfur and oxygen isotopic compositions of dissolved sulfate from a vertical water profile in the anoxic, hypersaline Orca Basin provides important constraints upon the provenance of the sulfate. Results support a dissolution origin of the brine from a Jurassic evaporitic salt. However, mass balance considerations and the observed minima in the vertical δ18O-sulfate profile at the brine-seawater interface are not reconcilable with only a simple two box brine-seawater mixing model. Instead, the data imply contributions from the oxidation of sulfides.

Seismic stratigraphy based chronostratigraphy (SSBC) of the Serbian Banat region of the Pannonian Basin

Seismic stratigraphy based chronostratigraphic (SSBC) analysis of the Serbian Banat region allows the delineation of the spatial and stratigraphic relationships of the generally regressive and shallowing upward Neogene depositional fill of a tectonically unstable central portion of the Pannonian Basin. When geometrically restored in time and space, the sediment dispersal directions, sediment source directions, types of sedimentation breaks and the tectonic events influencing basin evolution can be delineated. For such an analysis the time-transgressive lithostratigraphic units used in the neighbouring Hungarian part of the Pannonian Basin are conveniently introduced based upon their characteristic seismic facies and constrained borehole log records as mappable seismic stratigraphic sequence units, termed “seismic operational sequences”. The respective Neogene stage and operational sequence equivalents (Hungarian lithostratigraphic units or formations) are the Middle Miocene (Badenian, Sarmatian), Upper Miocene-Lower Pliocene (Pannonian-Endrod and Szolnok Formations; Pontian- Algyo and Ujfalu Formations and Lower Pliocene- Zagyva Formation) and Upper Pliocene-Quaternary (Nagyalfold Formation).SSBC analysis greatly assists in the geological constraint or “geovalidation” of interpreted seismic stratigraphic relationships and provides potentially critical insight into stratigraphic and structural problems of non-unique interpretations. In the specific case, using such an approach on previously unpublished regional seismic lines, SSBC analysis reveals that the Banat region has undergone structural inversion. This may be related to changes in local stress directions along strike slip faults, which initiated in earliest Late Miocene (Endrod Formation), culminating in the reverse tilting and incipient shortening of the western graben. Therefore during the time interval that the Badenian through Endrod sediments were deposited in the graben, autocyclic progradation initiated from the Kikinda Szeged High in the East followed by Szolnok, Algyo, Ujfalu and younger units prograding from the West as the central high uplifted relative to the graben. Such tectonic inversion has substantial hydrocarbon potential implications for exploration in the region.

Assessing Source Rock Maturity in Frontier Basins: Importance of Time, Temperature, and Tectonics

Rate constants are presently not established well enough to allow accurate theoretical quantification of organic matter evolution. Instead, a rapid empirical method for determining source rock maturity in relatively unexplored basins as a function of sediment age and thermal history is developed. From a compilation of published worldwide data, the oil threshold temperature may be empirically described in a non-Arrhenius relationship as a function of sediment age: T = 164.4 - 19.39 ln t, where T is threshold temperature in °C and t is time in 106 years. The depth to the oil ceiling in a sedimentary basin is then Doc = 100[(T - Ts)/(dT/dZ)], where Doc is depth in meters, T is derived from the preceding relationship, Ts is the average surface temperature, and dT/dZ is the geothermal gradient in °C/100 m. Similarly, the depth in meters to the oil floor, Dof, may be calculated from Dof = 100[(150 - Ts)/(dT/dZ)]. Sedimentary geothermal gradients can vary in time and space. These nonsteady-state and/or tectonically perturbed basin thermal histories are more complicated and must be considered in order to apply maturation models correctly. For sedimentary basins floored by oceanic crust less than 120 m.y. old, the geothermal gradient may be modeled as dT/dZ = 113/(K^radicalt), where K is the thermal conductivity. Basins in convergent tectonic settings have characteristic geothermal gradients that vary as a function of tectonic setting.