Source Rock Generation Research Articles

TECTONIC CONTROLS ON SEDIMENTATION AND MATURATION IN THE OFFSHORE NORTH PERTH BASIN

The major tectonic and stratigraphic elements of the offshore North Perth Basin have been delineated from regional BMR multichannel seismic reflection lines, together with industry seismic and well data. This analysis reveals that three sub- basins, the Edel, Abrolhos and Houtman Sub- basins, have formed as a result of three distinct episodes of rifting within the offshore North Perth Basin during the Early Permian, Late Permian and Late Jurassic respectively. During this period, rifting has propagated from east to west, and has culminated in the separation of this part of the Australian continent from Greater India.The boundaries between the sub- basins and many structures within individual sub- basins are considered to have been produced by strike- slip or oblique- slip motion. The offshore North Perth Basin is believed to be a product of transtension, possibly since the earliest phase of rifting. This has culminated in separation and seafloor spreading by oblique extension along the Wallaby Fracture Zone to form a transform passive continental margin.This style of rifting and extension has produced relatively thin syn- rift sequences, some of which have been either partly or completely removed by erosion. While the source- rock potential of the syn- rift phase is limited, post- rift marine transgressional phases and coal measures do provide adequate and relatively widespread source rocks for hydrocarbon generation. Differences in the timing of rifting across the basin have resulted in a maturation pattern whereby mature sediments become younger to the west.

Geochemical and Well-Log Investigation of an Actively Generating Source Rock Sequence: Waltman Shale, Wind River Basin, Wyoming: ABSTRACT

The Waltman Shale Member of the Fort Union Formation is a thick lacustrine shale deposited in the Wind River basin during Paleocene time. Available geologic and geophysical data suggest that the depositional conditions were relatively constant during deposition. These data also indicate that Lake Waltman was deep, with maximum water depths on the order of 500 ft. Sonic logs indicate that within deeper portions of the basin a velocity inversion is seen in the Waltman Shale Member, where the expected increase in acoustic velocity with depth due to compaction is not observed. An examination of available data reveals that this anomaly does not appear to be a result of either differences in organic matter content or mineralogy. The inversion appears to be most likely the result of overpressuring within the unit. The pyrolysis data indicate that the overpressuring within the unit. The pyrolysis data indicate that the overpressuring is associated with hydrocarbon generation and the inability of the system to effectively expel these hydrocarbons through a massive shale. The proposed relationship among the acoustic inversion, overpressuring, and active hydrocarbon generation is supported by the disappearance of the acoustic anomaly and the anomalous free hydrocarbon content in the vicinity of sandmore » layers in some of the wells examined. These sands allow for the expulsion or drainage of hydrocarbons from the shale and a return to normal pressures and a normal seismic velocity profile.« less

Modelling of hydrocarbon generation patterns in the Egersund Sub-Basin, North Sea

Hydrocarbon generation in the Egersund Sub-Basin has been modelled using empirical (Lopatin-type) and kinetic models and two prototypes of source rocks from wells 9/4-4 and 18/11-1. The amount of oil generated changes with initial heat flow, type of generation model, activation energies and source rock richness but does not follow a straightforward pattern. For example at lower thermal histories the amount of oil generated up to the Present changes exponentially with initial heat flow; variation due to change of activation energy distribution is linear to exponential, depending on heating rate; and variation due to source rock potential is linear. The empirical model for oil generation is less sensitive to temperature and more sensitive to time than the kinetic models and appears to overestimate the amount of oil generated at low heating rates but underestimates it at high heating rates. Hydrocarbon yields predicted by the empirical model are usually 4–5 times higher than those predicted by the kinetic models. For the drainage area of well 9/2-1 variation of initial heat flow between 1.15 and 1.25 hfu (heat flow units) results in a variation of output yield by a factor three. If a minimum oil saturation of the pore volume of 10% is required for long-distance migration, the empirical model should have resulted in accumulations or oil shows also in the marginal drainage areas. Although not unambiguous, the present hydrocarbon distribution can be explained best by generation from the kinetic model at thermal histories represented by initial heat flow between 1.20 and 1.30 hfu.

Sandstone Diagenesis and Source Rock Analysis of Cretaceous Blackleaf and Frontier Formations in Parts of Beaverhead, Madison, and Gallatin Counties, Montana: ABSTRACT

The mid-Cretaceous Blackleaf and Frontier Formations and equivalent strata in parts of Beaverhead, Madison, and Gallatin Counties, Montana, are composed of a marine and nonmarine sequence of clastic and carbonate rocks deposited along the western margin of the Western Interior seaway. At present seven dry holes have penetrated this sequence in southern Beaverhead County. Sandstones in the formations are diagenetically diverse and exhibit calcite, silica, sericite, zeolite, and clay mineral alteration. Alteration is more pervasive in the upper Blackleaf and lower Frontier Formations because of compositional heterogeneity and the presence of volcanic detritus. Lower Blackleaf alteration is less complex and is mainly due to quartz overgrowths and calcite cements. Porosity in lower Blackleaf quartz arenites in eastern Beaverhead, Madison, and Gallatin Counties has been reduced significantly by quartz overgrowths. Lower Blackleaf selitharenites in Beaverhead County contain calcite pore-filling cement but exhibit the best reservoir rock potential of both formations. Vitrinite reflectance ranges from a maximum of 3.62% for shales in the lower Blackleaf Formation in the Pioneer Mountains in northern Beaverhead County to a minimum of 0.43% for the lower Frontier Formation in the Lima Peaks area to the south. Maximum reflectance values are related to the proximity of Cretaceousmore » and Tertiary intrusive bodies. Reflectance values for all other samples within the study area lie just below or at the base of the oil generation window. Total organic carbon (TOC) values for both formations are higher in the Gallatin Range (maximum value equals 8.1%), but Rock-Eval pyrolysis data indicate generally poor source rock potential. Black shales of the lower Blackleaf and lower Frontier Formations provide the best source rock for generation of hydrocarbons.« less

Regional Hydrocarbon Generation, Migration, and Accumulation Pattern of Cretaceous Strata, Powder River Basin: ABSTRACT

A cell of abnormally high fluid pressure in the deep part of the Powder River basin is centered in an area where oil-generation-prone source rocks in the Skull Creek (oldest), Mowry, and Niobrara (youngest) formations are presently at their maximum hydrocarbon-volume generation rate. The overpressures are believed to be caused by the high conversion rate of solid kerogen in the source rocks to an increased volume of potentially expellable fluid hydrocarbons. In this area, hydrocarbons appear to be the principal mobile fluid species present in reservoirs within or proximal to the actively generating source rocks. Maximum generation pressures within the source rocks have caused vertical expulsion through a pressure-induced microfracture system and have charged the first available underlying and/or overlying sandstone carrier-reservoir bed. Hydrocarbons generated in the Skull Creek have been expelled downward into the Dakota Sandstone and upward into the Muddy Sandstone. Hydrocarbons generated in the Mowry have been expelled downward into the Muddy or upward into lower Frontier sandstones. Hydrocarbons generated in the Niobrara have been expelled downward into upper Frontier sandstones or upward into the first available overlying sandstone in the Upper Cretaceous. The first chargeable sandstone overlying the Niobrara, in ascending order, may be the (1) Shannon, (2) Sussex, (3) Parkman, (4) Teapo , or (5) Tekla, depending on the east limit of each sandstone with respect to vertical fracture migration through the Cody Shale from the underlying area of mature overpressured Niobrara source rocks. Vertical charge into each of the various carrier-reservoir sandstone units from their related source rock has been followed by a process of dominantly lateral updip migration within the carrier-reservoir bed toward sites of entrapment. Purely updip migration paths have been modified by both stratigraphic complexity and ground-water hydrodynamic flow. Stratigraphic-type traps terminating migration paths predominate on the north flank of the basin. Anticlinal traps predominate on the western and southern flanks. End_of_Article - Last_Page 856------------

Archaean metallogeny: A synthesis and review

Granitoid rocks interspersed with greenstone belts together comprise Archaean cratons throughout the world. The greenstone belts contain a wide variety of volcanic rocks which, despite cyclical variations in composition, generally change from ultramafic komatiites toward the stratigraphic base of the successions, upward through tholeiitic basalts and calc-alkaline andesites, to silici-alkalic rhyodacites toward the top. These extrusive rocks are intruded by rocks of a similar wide compositional range, which are probably comagmatic and subvolcanic to the former. The volcanic rocks are also intercalated with, and flanked by, volcaniclastic and distinctive immature sedimentary strata, including turbiditic greywacke and polymictic conglomerate. All are products of the prolonged volcanism that dominated Archaean supracrustal evolution and metallogenesis. Rare element pegmatites are associated with the Archaean granitic intrusions. Four important types of metalliferous ores, iron-manganese, nickel-chrome, gold-silver and copper-zinc occur in the greenstone belts, often co-regionally with one another in the same mining districts. Algoma type iron-formations of oxide, carbonate, silicate and sulfide facies occur throughout the volcano-sedimentary successions from base suggest common genetic processes for these ores. The Algoma type iron-formations are chemical sedimerare chromite deposits are restricted to the stratigraphically lower, ultramafic komatiites. Important gold ores are hosted primarily in the tholeiitic basalts, particularly where these are intercalated with ankeritic-pyritic chemical sedimentary strata, but smaller gold deposits are also known in stratigraphically lower ultramafic and higher felsic volcanic rocks. The largest massive base metal sulfide deposits occur in the stratigraphically higher felsic rhyodacitic members. The close spatial associations between deposits of these metals in Archaean rocks, particularly those of certain nickel, gold and base metal ores with iron-formation, together with their many similar geological characteristics, suggest common genetic processes for these ores. The Algoma type iron-formations are chemical sedimentary precipitates from ferruginous hydrothermal fluids that were periodically discharged on the sea floor during the prolonged Archaean subaqueous volcanism. The massive base metal deposits are of similar origin, essentially Cu-Zn-rich varieties of sulfide-facies iron-formation. The auriferous cherty, ankeritic or pyritic chemical sedimentary strata were also formed by similar sea floor exhalative hydrothermal activity. Although seldom of mineable gold content themselves, these constituted important, pre-enriched source rocks for later metamorphic generation of gold veins. Although many of the nickel sulfide and chromite bodies are of magmatic generation, others closely associated with iron-formation, and themselves delicately interbedded with cherty or talc-carbonate laminae, may be due to similar sea floor hydrothermal discharge that accompanied ultramafic extrusive activity. Considering their close spatial and genetic links, the occurrence of any one of these four types of deposit suggests the possibility of the others wherever the favourable Archaean host rocks are present. Different Archaean cratons however, have differing proportions of these four types of deposit, and of their distinctive host rocks. Greenstone belts in all cratons throughout the world contain the iron-formations and gold deposits. Greenstone belts of southern Africa and Western Austrialia, however, have more abundant ultramafic rocks and more important nickel-chrome deposits. Some of them may be older than comparable belts in Canada which contain more rhyodacitic rocks and more important copper-zinc ores. Some belts of Brazil and West Africa may be still younger, contain more pyroclastic-volcaniclastic rocks, lack both the nickel-chrome and copper-zinc ores, but contain important manganese in their iron-formations. These relations suggest worldwide diachroneity of Archaean greenstone belt generation, late-Archaean granitic orogeny and ensuing Proterozoic sedimentation.

A Novel Approach for Recognition and Quantification of Hydrocarbon Migration Effects in Shale-Sandstone Sequences

A detailed organic geochemical study of over 150 samples from two cores with a total combined length of 320 m (1,050 ft) through sequences of interbedded source rock-type shales (0.84% Rm maturity) and reservoir sandstones allowed recognition and quantitation of a number of migration effects. Detailed gas chromatography-mass spectrometry of steranes and triterpanes was used to insure that samples being compared to investigate migration effects contain organic matter of a similar type. Thin shales interbedded in sands and the edges of thick shale units are depleted in petroleum-range hydrocarbons to a much higher degree than the centers of thick shale units. For the alkanes, expulsion occurs with pronounced compositional fractionation effects: shorter chain length n-alkanes are expelled preferentially, and isoprenoid alkanes are expelled to a lesser degree than their straight-chain isomers. Based on material balance calculations, expulsion efficiencies were determined and found to be very high in certain instances. For thin interbedded shales, they decrease from about 80% around C15 to near zero in the C25+ region. There is no evidence for significant redistribution of steranes and triterpanes in the two sequences. Compared to C15 to C25 n-alkanes, they appear relatively immobile. The composition of the hydrocarbons impregnating parts of the reservoir sandstones is in agreement with expulsion occurring with pronounced fractionation based on molecular chain length. Hence, consideration of bulk expulsion efficiencies gives an unrealistic picture. Furthermore, the impregnation of a siltstone cap rock from an underlying hydrocarbon accumulation seems to have occurred by bulk-oil migration and without significant fractionation. The degree of hydrocarbon depletion of some of the shales of both sequences appears to be controlled by compaction, and the primary migration process appears to have occurred with chromatographic separation. The migration phenomena observed in both sequences lead us to propose that the main phase of expulsion can be preceded by an earlier sta e, during which the edges of thick shales and thin interbedded shales appear to be slowly and continuously depleted by the chromatographic processes. The composition of the hydrocarbon product accumulating in the reservoirs at this stage appears to be controlled primarily by physical processes rather than by the type and maturity of the organic matter in the generating source rock. By this mechanism, the origin of accumulations of light oils and gas-condensates in low mature sequences bearing predominantly terrestrial-derived organic matter can be explained. Finally, the migration effects documented in this study have some consequences for interpretation of geochemical data (e.g., the pristane/n-C17 ratio, a commonly accepted maturity parameter, has been shown to be also controlled by the degree of hydrocarbon expulsion).

Geochemical Surface Exploration for Hydrocarbons in North Sea

Approximately 350 sediment samples were taken in the central part of the North Sea, and hydrocarbon composition, methane (C1) and ethane (C2) yields, and carbon isotope composition of the methane (^dgr13C1) adsorbed in the sediment were determined. Methane generation or oxidation by bacterial activity can not be deduced from the analytical data. This fact is explained by the assumption of a threshold of methane concentration in sediments below which methane oxidizing bacteria are inactive, and that this threshold was not reached in the area investigated. Background gases in the surface sediments, characterized by low C1 yields and ^dgr13C1 > -37 ppt can be distinguished from thermal sediment g ses which are obviously derived from a hydrocarbon generating source rock in the subsurface. Source rock data, obtained from several wells situated on a cross section through the investigated area, were compared to the geochemical patterns of the sediment gases and used to define profile sections with (^dgr13C1 ^approx -40 ppt, yield C1 ^approx 217 ppb) and source rock properties (^dgr13C1 ^approx -33 ppt, yield C1 ^approx 24 ppb). An assessment of the source rock potential of the whole area under investigation has been carried out using the geochemical data of the surface sediment gases and taking into account that the C1 and C2 yields are generally higher in clay sediments than in sand samples. The number of producing wells was related to the number of all wells (271) in the areas characterized by surface geochemistry as good or poor source rock areas. The comparison showed that the relative number of producing wells is higher in areas geochemically characterized as good. Hydrocarbons in surface sediments of North Sea areas are not generated by bacteria at shallow depth; they are formed thermally from the organic carbon of deep source rocks and migrate to the surface. Therefore they are indicative of thermal hydrocarbon generation in the subsurface.

Abnormal Pressures Produced by Hydrocarbon Generation and Maturation and Their Relation to Processes of Migration and Accumulation: ABSTRACT

Abnormally high pore-fluid pressures may be produced as a result of hydrocarbon generation from organic matter (kerogen) contained in Contributing processes include: (1) collapse of rock matrix as overburden-supporting solid kerogen is converted to non-expelled fluid hydrocarbons and (2) volume increases produced by the conversion of kerogen to hydrocarbons. Overpressures may also be created by volume increases associated with the thermally progressive conversion of oil to wet gas/condensate and to dry gas within the pores of either the source rock or associated isolated hydrocarbon-saturated nonsource rocks. Generation-type overpressures may be reinforced or maintained by (1) fluid-volume expansions caused by higher temperatures associated with further bu ial and by (2) capillary entry pressure phenomena associated with expulsion/migration. The occurrence, maintenance, and degree of fluid overpressuring appear to be dependent on time, temperature, volume of kerogen/hydrocarbons undergoing transformation, and the relative isolation of the rocks with respect to regionally extensive high-permeability rocks. Regions of hydrocarbon-generation overpressure have been documented in several basins. They are present as vertically and laterally restricted or pods centering around actively generating source-rock units in basin-bottom positions. Hydrocarbons in most places appear to be the overpressuring fluid and the only initially producible fluid species present. Actively generating source rocks within the pressure cells may be characterized by (1) abnormally high electrical resistivities and abnormally low sound velocities. Resistivity increases may be caused by the replacement of conductive pore water with nonconductive hydrocarbons. Low sound velocities may be caused by (1) the replacement of higher velocity pore water with lower velocity hydrocarbons and (2) the effects of abnormal pressure on porosity enhancement or preservation through dilation or undercompaction. Overpressures produced by hydrocarbon generation are the primary motivating forces causing primary expulsion from source rocks to conventional reservoir rocks. These overpressures may cause the spontaneous hydraulic fracturing of a source rock. The process facilitates fluid expulsion and may also create an associated in-situ fracture-type reservoir. The process may also create far-reaching fractures which propagate upward or downward from the source rocks and control vertical migration through great thicknesses of seemingly impermeable confining strata. After active high-rate hydrocarbon generation has ceased in a basin, owing either to (1) decreases in temperature produced by uplift or lower heat flow or to (2) loss of generation capacity due to over-maturity, the associated pod of abnormally high pressures may be replaced by a pod of abnormally low pressures associated with closed apparent minimum potential energy (potentiometric) sinks. This stage of pressure evolution is controlled by the imbibition of water into the initially overpressured area of high hydrocarbon saturation. Contributing processes may include (1) the volume contraction of pore-fluid hydrocarbons caused by decreases in temperature, (2) the establishment of topographically introduced hydrodynamic conditions, (3) the solution-diffusion of hydrocarbons (prima ily methane) outward from the system, and (4) the capillary imbibition of water. Significant accumulation of hydrocarbons in highly unconventional basin-bottom positions may occur in either the overpressured or underpressured stages of basin hydrocarbon generation-migration evolution. When water is imbibed into the original site of hydrocarbon generation and overpressure, and the associated unstable localization of hydrocarbon saturation has been dissipated and destroyed during an associated stage of underpressure, pore pressure conditions will eventually return to near normal states. End_of_Article - Last_Page 2467------------

Examples of Abnormal Fluid Pressure Produced by Hydrocarbon Generation: ABSTRACT

Abnormally high pore fluid pressures may be produced as a result of hydrocarbon generation from organic matter (kerogen) contained in rocks. Contributing processes include: (1) collapse of rock matrix as overburden-supporting solid kerogen is converted to non-expelled fluid hydrocarbons and (2) volume increases produced by the conversion of solid kerogen to hydrocarbons. Hydrocarbon generation overpressures exist in the following basins: Williston, Powder River, Anadarko, Delaware, Uinta, and San Joaquin. The phenomenon probably also exists elsewhere. These regions of overpressure represent vertically and laterally restricted or pods in which hydrocarbons are the overpressuring fluid and the only initially producible fluid species present. The pressure cells center around actively generating source-rock units. Actively generating source rocks within the pressure cells may be characterized by abnormally high electrical resistivities and abnormally low sound velocities. Resistivity increases may be caused by the replacement of conductive pore water with non-conductive hydrocarbons. Low sound velocities may be caused by: (1) the replacement of higher velocity pore water with lower velocity hydrocarbons and (2) the effects of abnormal pressure on porosity enhancement or preservation through dilation or under-compaction. Hydrocarbon generation overpressures lead to the spontaneous hydraulic fracturing of the source rock and may create associated fracture-type reservoirs. They may also create fractures which propagate upward or downward from the source rocks and control vertical migration through large thicknesses of seemingly impermeable confining strata. When a source rock ceases to generate, abnormal pressures may decay, resulting in normal or subnormal pressure conditions. End_of_Article - Last_Page 749------------

THE EFFECTS OF CONTINENTAL DRIFT ON THE PETROLEUM GEOLOGY OF WESTERN INDONESIA

The traditional concept of western Indonesia as a sinuous island are is probably incorrect. Instead, western Indonesia seems to consist of a series of largely straight segments that are sometimes offset and that may change direction abruptly in specific hinge line areas. The segments, especially in these hinge line areas, are cut by a system of north-northeast trending transverse faults. This fault system (Bantam trend) is noted forremarkable consistency in the direction of strike,wide distribution,variety of amount and direction of movement,occasional volcanism, andvaried activity at different times during the Cenozoic period or before, probably diminishing in a northeasterly direction.The segmentation and associated transverse fault system of western Indonesia may be caused by the Australian plate and its fracture zones (extensions of "transform" faults?) and north-northeast trending Indian Ocean morphological features, being thrust beneath the Indonesian continental area. The Bantam (transverse) fault trend may have affected the petroleum geology of western Indonesia in at least two ways:it may have been responsible for the division of the backdeep and perhaps the interdeep into a series of highs and lows, resulting in silled conditions during deposition and therefore, source rock generation;it probably provided the usual north-northeast trending oil and gas producing anticlines in northern Java and in the Java Sea. As the less explored interdeep basins are closer to the edge of the subduction zone, the Bantam trend faults are likely to be particularly prominent there, a factor that should affect exploration programmes in this region.