Seismic Bright Spots Research Articles

New prospective gas plays in pliocene sands, offshore Nile Delta Basin: A case study from Kamose-1 well at North Sinai Concession, Egypt

The Pliocene sands in the offshore parts of the Nile Delta Basin represent significant gas reservoirs. Several targets have been recently discovered in different places along the offshore Nile Delta Basin. The present work focuses on the possibility of discovering new natural gas resources at North Sinai Concession through an integrated method between seismic and well logs data interpretations. A case study from Kamose-1 Well at Kamose Field, North Sinai Concession has been examined. The quantitative explanation for the well log data of Kamose-1 Well has revealed two zones expected to be gas-bearing zones. These intervals are located between depths; 1174–1186 m (zone I) and between 1429 and 1503 m (zone II). The calculated petrophysical parameters for these zones exhibit low water saturation (24–43%), high effective porosity (30–31%), low shale volume (21–26%), high total porosity (37–38%) and low bulk volume of water (0.07–0.12). Zone II (74 m thick) shows a clear bright spot on the seismic profiles covering the study area, while, zone I is beyond seismic resolution (12 m thick). Several seismic bright spots have been outlined on the seismic data nearby Kamose-1 Well. The comparison between the newly outlined seismic bright spots with the drilled one in Kamose-1 Well exhibited a great similarity. Accordingly, these prospects are expected to be new targets for gas exploration in the Pliocene succession at North Sinai Concession.

Characteristics and geological properties of seismic bright spots in the Permian carbonate deposit, Changhsing Formation, Longgang Area, Northeast Sichuan Basin, China

Bright spot seismic reflections in carbonate strata are often correlated with effective reservoirs. Widely distributed bright spot reflections have been found in the Permian Changhsing Formation in the Longgang area, Northeast Sichuan Basin, China. However, there is still a lack of systematic understanding of the types, distribution, and geological origin of these bright spot reflections in the Longgang area. Integrated subsurface analysis has revealed that bright sport characteristics represent a combination of strong troughs and peaks, with thicknesses up to 30 ms of two-way time and have a shape like “a string of pearls” in the seismic section. The bright spots located in the platform margin show a NW-trending intermittent strip-like distribution, while in the open platform they are scattered. Bright spots are mainly distributed at the top of the Changhsing Formation and form in the platform margin and open platform environments. The distribution of bright spots has a good coupling with reefs and shoals located in the high paleogeographic regions of the Upper Changhsing Formation. The integrated analysis based on cores, thin sections, and geophysical data reveals that the “pearl-like” bright spots are indicators of needle-like dissolution pores, small karst caves, fracture–vug cystic caves and piebald karst fracture cave systems resulting from eogenetic karstification. Large-scale regression of sea level, which occurred at the end-Permian section, is the main control for reefs and shoals located in the exposed paleohighs suffering from eogenetic karstification. Exploration based on the location of bright spot seismic reflections may be a favorable target for hydrocarbons.

Seismic Bright spots in Qiangtang Terrane from deep reflection surveying

Seismic Bright spots in Qiangtang Terrane from deep reflection surveying

Geological origins of seismic bright spot reflections in the Ordovician strata in the Halahatang area of the Tarim Basin, western China

Anomalously high-amplitude seismic reflections are commonly observed in deeply buried Ordovician carbonate strata in the Halahatang area of the northern Tarim Basin. These bright spots have been demonstrated to be generally related to effective oil and gas reservoirs. These bright spot reflections have complex geological origins, because they are deeply buried and have been altered by multi-phase tectonic movement and karstification. Currently, there is no effective geological model for these bright spots to guide hydrocarbon exploration and development. Using core, well logs, and seismic data, the geological origins of bright spot are classified into three types, controlled by karstification, faulting, and volcanic hydrothermal activity. Bright spots differing by geological origin exhibit large differences in seismic reflection character, such as reflection amplitude, curvature, degree of distortion, and the number of vertically stacked bright spots in the seismic section. By categorizing the bright spots and the seismic character of the surrounding strata, their geological origins can after be inferred. Reservoirs formed by early karstification were later altered by epigenetic karstification. Two periods of paleodrainage further altered the early dissolution pores. In addition, faults formed by tectonic uplift also enhanced the dissolution of the flowing karst waters. Some reservoirs were subsequently altered by Permian volcanic hydrothermal fluids.

Generation, migration, entrapment and leakage of microbial gas in the Dutch part of the Southern North Sea Delta

Understanding the shallow gas system is critical for assessing its potential as an energy source, for evaluating the possible hazard of shallow gas for drilling and wind farm locations and for evaluating the effect of gas emissions at the seabed on marine ecosystems and climate. This paper presents the key elements and processes of the microbial shallow gas system in the Plio-Pleistocene Dutch Southern North Sea Delta based on recent findings from different projects. Geochemical and carbon isotopic composition of shallow gas occurrences in the delta are indicative of a microbial origin of the gas. Shallow gas mainly occurs in stratigraphic traps and stacked anticlinal structures above salt structures, as indicated by identified direct hydrocarbon indicators on seismic, such as bright spots. Organic matter in the delta deposits is of predominantly land plant origin with TOC values varying between <1% and 5%, and mostly between 1 and 2%. Simulations of temperature and burial history in combination with dedicated 1D simulations of microbial gas generation reveal that gas generation in the delta started in Early Pleistocene Calabrian times and is still ongoing. Simulated volumes of gas generation are more than enough to fill published estimated volumes of shallow gas prospects in the delta. The geometry of the E-W prograding delta sequences and the close interbedding of interglacial silty/sandy and glacial clayey/silty sediments focus gas migration updip through the foresets towards the topsets of the delta sequences, and ultimately into the anticlinal stacked traps. Grain-size based calculations of the permeability and capillary seal capacity of clayey/silty seals of the stratigraphic and anticlinal traps provided first estimates of permeability values ranging from 2.8E-20 m2 to 1.1E-18 m2 and capillary seal capacity values between 10m and 24m. Comparison of gas column heights derived from grain-size based calculations, cross plots of neutron and density logs and pressure measurements, with trap heights derived from seismic bright spots suggests that many traps in stacked bright spots are not filled to structural spill point. This suggests that filling of the stacked reservoirs is not related to fill-spill migration, but rather to leakage through the top seal. In absence of fault and fracture zones crossing the seal, this leakage is related to the capillary seal capacity and permeability of the top seal. The leakage extends, locally, to the seabed. The microbial gas system in the delta today is a highly dynamic system driven by ongoing burial of the delta sediments and microbial gas generation. © 2018

Miocene magmatism in the Western Nyainqentanglha mountains of southern Tibet: An exhumed bright spot?

The Western Nyainqentanglha (WNT) mountain range of south-central Tibet predominantly comprises granitoids that intrude into metasedimentary strata, and was exhumed from ~15 to 20km depth in the footwall of the Yangbajain graben during the late Neogene. The range provides a rare exposure of deeper crustal levels of Tibet, which can be used to gain insight into processes that may be occurring beneath the plateau. Field, petrological, thermobarometric and U–Pb geochronological analyses are applied to determine the magmatic and metamorphic history of the WNT, revealing three tectonothermal events: (1) 213–201Ma magmatism and amphibolite-facies metamorphism associated with north–south Lhasa terrane accretion, (2) 140–52Ma magmatism resulting from subduction of Neotethys preceding India–Asia collision, and (3) 25–8Ma magmatism that we suggest to have resulted from partial melting of the thickened Tibetan plateau crust. The latter is correlated with seismic bright spots imaged in the region at ~15–18km depth, indicating that restricted partial melting at mid-crustal levels may have been ongoing since 25Ma, in accordance with observations from xenolith data and predictions made by thermal modelling of thickened crust.

Ductility contrast in pelitic schist: microtectonic evidence for seismic bright spots

Ductility contrast in pelitic schist: microtectonic evidence for seismic bright spots

Gas hydrate and free gas petroleum system in 3D seismic data, offshore Angola

Evidence of gas hydrate and free gas occurrences in a 3D seismic volume from the West-Central Coastal Province of the Congo Fan, offshore Angola, illustrates all the components of a complete petroleum system. Analysis and interpretation are based on the information in attributes calculated from three 3D time-migrated common-angle seismic volumes; the attributes include seismic amplitude, spectral components, dip magnitude, amplitude variation with angle, and instantaneous frequency. The source is organic-rich muds associated with late Cretaceous to early Tertiary channels, the migration paths are along growth faults, and the traps are partly defined by the gas hydrate stability zone (for the gas hydrate), partly by the bottom-simulating reflector (for the subhydrate free gas), and partly by faults (for both). The spatial distribution of free gas is further supported by the associated seismic bright spots, and also by the attenuation of high frequencies of P-waves that traverse the gas-saturated zone. Locally higher temperatures, associated with upward fluid circulation along fault zones, facilitate gas transmission through the gas hydrate and forms gas chimneys that extend to the sea floor.

Dim spots: Opportunity for future hydrocarbon discoveries?

When water in the pores of a reservoir rock is replaced by hydrocarbon, the acoustic impedance universally reduces. When the reservoir becomes more porous, the impedance reduces further. When net-to-gross ratio (for most reservoirs) increases, the average impedance of the reservoir reduces further still. Thus low impedance is prospective. If the prospective reservoir is encased in higher-impedance rock, this produces more contrast and thus a seismic bright spot. If the prospective reservoir is encased in lower-impedance rock, this produces less contrast and thus a seismic dim spot.

Three-dimensional seismic geomorphology and analysis of the Ordovician paleokarst drainage system in the central Tabei Uplift, northern Tarim Basin, western China

High-quality three-dimensional seismic data acquired in the central Tabei Uplift, Tarim Basin, western China, provide a rare opportunity to characterize in exceptional detail the three-dimensional geomorphology of a deeply buried (5500–6500 m [18,045–21,325 ft]) Ordovician unconformity and the related paleokarst drainage system. An integrated approach was applied that emphasized integration of seismic data with available conventional core, wireline logs, and age-equivalent outcrops. The exceptional quality of the seismic data allowed a seismic detection limit of karstified features of less than 75 75 m (246 246 ft) horizontally and 6 m (20 ft) vertically. Interpreted geomorphologic and depositional elements include fluvial channels and canyons, fluvial valleys, sinkholes, and tower karsts and hills. The modern tower karst-drainage system in Guilin, China, is very similar to the mapped Ordovician karst-drainage system and is used as a modern analog. The interaction between the surface karst-drainage system and the shallow-subsurface cave-passage system is evidenced by the observation that surface canyons appear to initiate in areas associated with intense sinkhole development. Also, surface river valleys tend to correspond to dip-oriented surface depressions partly related to near-surface cave collapse. During burial into the deeper subsurface, the combination of intrastratal collapse (karstified strata) and suprastratal collapse (postkarst-deposited strata) created large damage zones hundreds of meters thick and kilometers wide. Coalesced-collapsed paleocave systems can be interpreted from the unique circular pattern of faults (observed in map view) that are associated with seismic bright spots.

Characterizing seismic bright spots in deeply buried, Ordovician Paleokarst strata, Central Tabei uplift, Tarim Basin, Western China

Anomalous seismic-amplitude bright spots are a common feature in deeply buried (5500 to 6500 m) Ordovician limestone strata in the Central Tabei Uplift area of the Tarim Basin in northwest China. Those anomalies have proven to be useful indicators of reservoir quality. The bright spots as seen on seismic data are tied to high-gamma ray, low-velocity zones in wireline logs, and correspond to clastic cave sediment-fills in the host limestone in core. Synthetic seismic models confirm this relationship between seismic bright spots and cave-sediment fills. A seismic traveltime map of the top Ordovician unconformity illustrates erosional topography and seismic geomorphologic patterns associated with the unconformity with numerous sinuous fluvial channels and canyons, fluvial valleys, sinkholes, and tower karsts and hills. A mature surface drainage system interacted with a near-surface karst system and allowed terrigenous sediments to enter an underground cave system. Karst-related bright spots probably correspond to paleocaves that initiated along an early regional fracture network and later were enhanced and altered by additional discharges in the surface drainage system. A few examples of bright spots are interpreted to be related to postkarstification faults that might have had a hydrothermal origin. Distribution of bright spots provides a useful reference in mapping regional collapsed-paleokarst systems. Bright spots typically are associated with circular and linear faults, and V-shaped depression patterns are related to a collapsed paleocave complex. Seismic-scale mapping and visualization of the paleokarst system can be achieved by interpreting circular and linear faults using continuity attributes and by analyzing the relationship among faults, local depressions, and bright spots.

Geochemistry, origin, and deep-water exploration potential of natural gases in the Pearl River Mouth and Qiongdongnan basins, South China Sea

More than 10 gas pools have been discovered since 1983 in the shallow-water region of the Pearl River Mouth (PRM) Basin and the Qiongdongnan (QDN) Basin, offshore South China Sea. Gases produced from the QDN basin are characterized by high contents of benzene and toluene and relatively heavy 13C2 values (25 to 27). The associated condensates have a high abundance of bicadinanes and oleanane, providing a good correlation with the coal-bearing sequence of the Oligocene Yacheng Formation in the basin. In contrast, the gases from the PRM basin contain lower amounts of benzene and toluene and lighter 13C2 values (24 to 34). Widely variable concentrations of bicadinane and oleanane were identifiied from the associated condensates, which can be mostly correlated with the lower Oligocene Enping Formation source rocks formed in a swamp to shallow lake environment. Oil-cracked gases sourced from the Eocene oil-prone source rock may also provide some contribution to the PRM basin gases. The available geochemical data indicate that both the Yacheng and Enping formations contain mainly type III and II2 kerogens with dominant gas potential. Regional geological information indicates that the deep-water regions of the two basins share the same hydrocarbon source sags with the shallow-water areas, and they developed massive sandstone reservoirs during the Oligocene and Miocene. Fluid-flow modeling results show that the deep-water regions were on the pathway of lateral migrating gases, and the interpreted reservoirs in these zones have developed abundant seismic bright spots, which may reflect the presence of gas. The deep-water regions of the offshore South China Sea are believed to have great gas exploration potential.

The role of crustal fluids in the tectonic evolution of the Eastern Goldfields Province of the Archaean Yilgarn Craton,Western Australia

Gold deposits in the Archaean Eastern Goldfields Province in Western Australia were deposited in greenstone supracrustal rocks by fluids migrating up crustal scale fault zones. Regional ENE-WSW D2 shortening of the supracrustal rocks was detached from lower crustal shortening at a regional sub-horizontal detachment surface which transects stratigraphy below the base of the greenstones. Major gold deposits lie close to D3 strike slip faults that extend through the detachment surface and into the middle to lower crust. The detachment originally formed at a depth near the plastic-viscous transition. In orogenic systems the plastic-viscous transition correlates with a low permeability pressure seal separating essentially lithostatic fluid pressures in the upper crust from supralithostatic fluid pressures in the lower crust. This situation arises from collapse in permeability below the plastic-viscous transition because fluid pressures cannot match the mean stress in the rock. If the low permeability pressure seal is subsequently broken by a through-going fault, fluids below the seal would flow into the upper crust. Large, deeply penetrating faults are therefore ideal for focussing fluid flow into the upper crust. Dilatant deformation associated with sliding on faults or the development of shear zones above the seal will lead to tensile failure and fluid-filled extension fractures. In compressional orogens, the extensional fractures would be subhorizontal, have poor vertical connectivity for fluid movement and could behave as fluids reservoirs. Seismic bright spots at 15–25 km depth in Tibet, Japan and the western United States have been described as examples of present day water or magma concentrations within orogens. The likely drop in rock strength associated with overpressured fluid-rich zones would make this region just above the plastic-viscous transition an ideal depth range to nucleate a regional detachment surface in a deforming crust.

Natural gas hydrates on the continental slope off Pakistan: constraints from seismic techniques

In October 1997, new vertical-incidence, wide-angle reflection and refraction data were obtained on the convergent continental margin off Pakistan. The most prominent feature across the entire margin into the Gulf of Oman is a bottom simulating reflector (BSR). This reflection generally coincides with the depth predicted for the base of the gas hydrate stability field. On the accretionary prism the BSR is a clear reflection with inverse polarity, and strata beneath the BSR show enhanced reflectivities, suggesting that gas is trapped between stratigraphic layers. Joint inversion of wide-angle and refraction data yielded a ∼200 m thick low-velocitiy zone at BSR depth. Using the Born expansion, band-limited impedance logs were calculated from deconvolved and scaled single-channel data, suggesting that velocity at the BSR drops by ∼200 m s−1. Some 40 km towards the coast, CDP4400 of Minshull & White (1989) yielded a velocity inversion of about 600 m s−1, indicating that BSR characteristics vary significantly across the continental margin off Pakistan. Beneath the Gulf the BSR was sampled at a similar subbottom depth, where seismic bright spots occur. However, its reflection amplitude is unusually low and seismic stratigraphy beneath the BSR did not show enhanced reflectivities. Joint traveltime inversion of wide-angle and refraction data revealed continuously increasing velocities, suggesting that an adequate supply of methane for forming free gas and gas hydrate is mainly confined to sediments incorporated into the accretionary prism. We believe that these variations are related to an evolutionary process due to tectonic uplift and ongoing sedimentation as sediments are incorporated into the accretionary prism. Sedimentation and uplift produce an upward migration of isotherms and therefore lead to a dissociation of gas hydrates, which releases methane gas and thus causes an enhancement of the BSR. Our survey imaged BSR properties of the abyssal plain and within the first slope basin at water depths of 3300 m and 2490 m, respectively. CDP4400 was located farther landwards at a depth of 1730 m within a buried and uplifted and hence more evolved slope basin. We therefore suggest that, in the accretionary wedge, tectonic uplift and ongoing sedimentation cause hydrate recycling by upward migration of the stability field. In addition, tectonic dewatering may accumulate hydrate at the base of the stability field by advection of methane from depth.

Measuring the seismic properties of Tibetan bright spots: Evidence for free aqueous fluids in the Tibetan middle crust

Seismic bright spots are commonly interpreted to mark fluid concentrations, but their nature (melt or aqueous) is usually inferred only from circumstantial evidence of the geologic setting. A band of bright spot reflections has been imaged by Project INDEPTH (International Deep Profiling of Tibet and the Himalayas) at about 15 km depth along 150 km of the northern Yadong‐Gulu rift, southern Tibet. We use INDEPTH three‐component wide‐angle seismic data to measure seismic velocities at the bright spot reflector, and theoretical rock physics bounds to constrain the nature of the fluids. Merging of data from multiple bright spots allows us to use a one‐dimensional approximation. Travel time modeling yields average P and S velocities for the upper crust above the bright spots of 5.3±0.2 and 3.2±0.2 km s−1, respectively. Reflection‐amplitude variation with offset (AVO) modeling constrains the P and S velocities of the bright spots to 3.0±0.8 and 1.6±0.8 km s−1, respectively. Multiple modeling procedures suggest these velocities are not model dependent. Our results imply that of the order of 10% volume of free aqueous fluids in the Tibetan middle crust produces the observed bright spot reflections. The presence of relatively large quantities of free aqueous fluids, presumably mostly saline supercritical H2O, does not preclude the presence of melt but does constrain the maximum temperature at the bright spots to the wet granite solidus (about 650°C) and thus the maximum surface heat flow to ≤110 mW m−2. The observed bright spots can alternatively be explained as a result of transient flow of aqueous fluids through a lower temperature and lower heat flow southern Tibetan crust.

Abstract: Seismic Bright Spots Sugguest Large Hydrocarbon Accumulation Trends in the Deep Gulf of Mexico

Abstract: Seismic Bright Spots Sugguest Large Hydrocarbon Accumulation Trends in the Deep Gulf of Mexico

Petroleum Geology of the Deltaic Sequence, Rio Del Rey Basin, Offshore Cameroon

The Rio Del Rey Basin of offshore Cameroon comprises the easternmost portion of the Niger delta complex. In the delta flank setting, the overall package is thinner and stratigraphic correlation simpler than in the depocenter to the west so the distribution of reservoirs and seals is well defined. Productive reservoirs are shallow, typically less than 2000 m, and mostly hydropressured to slightly overpressured. Reservoir properties generally are excellent and seismic bright-spot technology has played a major role in exploration and development efforts. Rio Del Rey is characterized by three main structural styles from north to south: (1) detachment-based growth faults, (2) highly faulted, mobile shale cored domes and ridges, and (3) toe thrusts and folds. Virtually all individual traps are fault dependent and lack of sufficient internal seals in sand-rich wave-dominated deltaic sequences can limit the objective window. Most fields are downthrown fault traps in which hanging-wall reservoirs are effectively sealed against older prodelta and marine shales. All large oil fields are located in the dome and ridge province and are potential analogs for similar structures now being explored on the upper slope off Nigeria. The best quality source rocks identified thus far are Paleocene to Eocene marine shales. Oilmore » vs. gas distribution in the basin is a function of variations in the kerogen type, organic richness, and maturity of these source rocks. A significant quantity of the gas found to date in Rio Del Rey is biogenic in origin. The Cameroon charge model may aid in better understanding hydrocarbon distribution on offshore Nigeria, where the Miocene deltaic section is much thicker and potential Paleocene-Eocene source rocks likely are unpenetrated. The shallow deltaic play in Cameroon is relatively mature. Future exploration likely will focus on identifying deeper, nonbright-spot-supported opportunities including potential stratigraphic traps.« less

To: “Prestack amplitude analysis methodology and application to seismic bright spots in the Po Valley, Italy” by Alfredo Mazzotti (GEOPHYSICS, 55, 157–166, February 1990)

In this paper, Figures 1a and 1b are incomplete. The correct figures are shown below.

Prestack amplitude analysis methodology and application to seismic bright spots in the Po Valley, Italy

The amplitude‐versus‐offset (AVO) characteristics of three separate bright spots on the same seismic section are analyzed. One of the bright spots results from a water‐bearing gravel layer, and the others correspond to gas‐saturated sandy beds. The amplitude analysis includes reflections from the entire range of incidence angles available from the survey; for the shallower amplitude anomaly, these angles reached values up to 66°. Extension of the analysis to longer offsets is aimed at detecting possible critical‐angle phenomena in order to reduce the uncertainty when the zero‐offset reflection’s polarity is unknown. The reflection from the gravel layer has this property. Its amplitude exhibits an initial decrease followed by a sudden rise in the AVO trend due to critical reflection and head waves. The gas‐related anomalies have a much different AVO characteristic, one in which the amplitude increases with offset distance. Two seismic events located above the bright spots were also investigated to further verify the validity of the seismic amplitude processing. The AVO trends of the three bright spots and of the two reference levels were compared with analogous trends of synthetic seismograms that were computed from models derived from borehole data.

South Tisdale Gas Field, Sacramento Basin, California: Discovery by Use of Seismic Amplitude Variation with Offset Analysis: ABSTRACT

The South Tisdale gas field is located on the eastern flank of the central Sacramento basin of California. The field was discovered in June 1986 by Nahama and Weagant Energy Company using seismic data concentrating on analysis of amplitude variation with offset in conjunction with subsurface data. The producing intervals, the Upper Cretaceous Forbes sands and the G zone Guinda sands, were deposited as fine-grained turbidite sands bounded by shales. Seismic bright spots observed in the Forbes interval can be caused by several materials, ranging from bentonites to gas sands. Amplitude-with-offset analysis was employed to high grade the bright spots to locate the gas response signature. The discovery well was drilled between two dry holes, 2000 ft apart, where an amplitude-with-offset seismic anomaly developed in the lower Forbes interval. All subsequent development wells have been drilled on such anomalies, resulting in four successful wells and one in progress as of this writing. Synthetic seismograms were made from each well to integrate the subsurface data into the seismic data. Additional proprietary seismic lines have been acquired and reviewed, revealing more anomalies to be tested. The amplitude-with-offset analysis, integrated with subsurface data, has resulted in a significant gas discovery.