Robert E. Mattick

Regional Geologic Framework Off Northeastern United States

Six multichannel seismic-reflection profiles taken across the Atlantic continental margin off the northeastern United States show an excess of 14 km of presumed Mesozoic and younger sedimentary rocks in the Baltimore Canyon trough and 8 km in the Georges Bank basin. Beneath the continental rise, the sedimentary prism thickness exceeds 7 km south of New Jersey and Maryland, and it is 4.5 km thick south of Georges Bank. Stratigraphically, the continental slope--outer edge of the continental shelf is a transition zone of high-velocity sedimentary rock, probably carbonate, that covers deeply subsided basement. Acoustically, the sedimentary sequence beneath the shelf is divided into three units which are correlated speculatively with the Cenozoic, the Cretaceous, and the Jurassic-Triassic sections. These units thicken offshore, and some have increased seismic velocities farther offshore. The uppermost unit thickens from a fraction of a kilometer to slightly more than a kilometer in a seaward direction, and velocity values range from 1.7 to 2.2 km/sec. The middle unit thickens from a fraction of a kilometer to as much as 5 km (northern Baltimore Canyon trough), and seismic velocity ranges from 2.2 to 5.4 km/sec. The lowest unit thickens to a maximum of 9 km (northern Baltimore Canyon), and velocities span the 3.9 to 5.9-km/sec interval. The spatial separation of magnetic and gravity anomalies on line 2 (New Jersey) suggests that in the Baltimore Canyon region the magnetic-slope anomaly is due to edge effects and that the previously reported free-air and isostatic gravity anomalies over the outer shelf may be due in part to a lateral increase in sediment density (velocity) near the shelf edge. The East Coast magnetic anomaly and the free-air gravity high both coincide over the outer shelf edge on line 1 (Georges Bank) but are offset by 20 km from the ridge on the reflection profile. Because the magnetic-slope-anomaly wavelength is nearly 50 km across, a deep source is likely. In part, the positive free-air gravity anomaly likewise may represent the significant lateral density increase within the sedimentary section to ard the outer edge of the shelf.

Correlation of Seismo- and Magnetostratigraphy in Southeastern Hungary

Correlation of results from magnetostratigraphic and seismic-reflection studies indicate that the Pannonian Basin, during the postrift phase of its evolution (middle Miocene to present), became filled by sediments of southward and eastward prograding deltaic wedges.

Structure of the Békés Basin Inferred from Seismic Reflection, Well and Gravity Data

The Bekes basin (areal extent 3900 km2) is a northwest-trending, Neogene basin located in southeast Hungary. The basin contains over 6500 m of synrift and postrift sedimentary fill. Middle Miocene synrift deposits are relatively thin and no Paleogene rocks have been reported to be present. The prerift section (basement) is composed of Mesozoic carbonate and clastic rocks and Paleozoic and older volcanic, igneous and metamorphic rocks. The Mesozoic rocks represent dominantly shallow-water environments and are up to 5000 m thick in the Bekes-Doboz Mesozoic trough and 2000 m thick in the Battonya-Pusztafoldvar Mesozoic trough.

Sequence Stratigraphy of the Békés Basin

The 6500-m-thick section of Neogene and Quaternary fill in the Bekes basin (3900 km2) reflects a normal cycle of rift-related sedimentation that began in late Badenian time (16.5 Ma) when marine waters were shallow (<200 m). By late Sarmatian or early Pannonian time (12−10 Ma), lacustrine conditions prevailed and water depths in the basin increased as a result of basin subsidence rates that greatly exceeded sedimentation rates. The latest history of the basin, from approximately 5.78 Ma, reflects continuously shallowing waters, which resulted from sedimentation rates that were generally higher than basin subsidence rates. By late Pannonian time (4.25 Ma), water depths in the lake were 200–400 m, and continued shoaling culminated in the eventual disappearance of the lake in Pliocene time.

Structure, Stratigraphy, and Petroleum Geology of the Little Plain Basin, Northwestern Hungary

The basement of the Little Plain (Kisalfold) basin is composed of two parts: an eastern part comprised of folded and overthrusted Triassic and Paleozoic rocks of the Pelso block (Transdanubian Central Range) compressed in the Early Cretaceous, and a western part consisting of stacked nappes of the Austroalpine zone of Paleozoic rocks, significantly metamorphosed during Cretaceous and later compression, overriding Jurassic oceanic rift-zone rocks of the Penninic zone. The evolution of the basin began in the late Karpatian-early Badenian (middle Miocene) when the eastern part of the basin began to open along conjugate sets of northeast- and northwest-trending normal faults. Neogene rocks in the study area, on the average, contain less than 0.5 wt. % total organic carbon (TOC) and, therefore, are not considered effective source rocks. Locally, however, where TOC values are as high as 3 wt. %, significant amounts of gas may have been generated and expelled. Although potential stratigraphic traps are numerous in the Neogene section, these potential traps must be downgraded because of the small amount of hydrocarbons discovered in structural traps to date. With the exception of the Cretaceous, the Mesozoic section has not been actively explored. Large anticlinal and overthrust structures involving pre-Cretaceous strata remain undrilled.

A Geophysical Study of North Park and the Surrounding Ranges, Colorado

A geophysical study in the North Park basin and surrounding mountains, Colorado illustrates the structural relationship of various sedimentary, metamorphic, and igneous rock units. Bouguer anomalies from 1330 gravity stations range from −210 mgal over Precambrian metamorphic rocks in the mountains to −260 mgal in the Walden syncline and —280 mgal in the North Park syncline. Steep gradients delineate a fault which strikes west-northwest along the north flank of the North Park syncline. Two models fitted to the gravity data show 1 to 2 km relief on this steeply dipping fault. Density contrasts between Precambrian metamorphic and igneous rocks produce anomalies of as much as 25-mgal amplitudes in the Park and Medicine Bow Ranges. A 30-km-long seismic refraction profile, parallel to the most negative Bouguer anomaly values in the North Park basin, shows velocities increasing from 2.5 to 3.4 km/sec within Tertiary rocks at depths ranging from 1.2 to 2.0 km. Mesozoic sedimentary rocks have a velocity of 4.0 to 4.5 km/sec, a very high velocity in view of the predominance of Upper Cretaceous rocks. Precambrian basement with a velocity of 6.25 km/sec underlies the profile at depths ranging from 3.5 to 4.5 km. Strong second arrivals across the profile, observed at distances of more than 14 km from the shotpoints and interpreted as SP reflections, verified the refraction model. An aeromagnetic survey shows numerous anomalies ranging from 100 to 200γ in the Park and Rabbit Ears Ranges and in the Never Summer Mountains, to 400γ in the Front Range, and to 1200γ over the Medicine Bow Range. Positive anomalies in the Park, Medicine Bow, and Front ranges overlie metamorphic rocks. Magnetic and gravity data suggest that the Never Summer Mountains are separated from the Front Range by a north-trending, steeply east-dipping reverse fault, extending beneath the Front Range along the Colorado River valley. The magnetic data indicate that this fault may connect with a possible fault that is parallel to the Laramie River valley. In the Rabbit Ears Range, a series of magnetic anomalies show that igneous rocks are present in the eastern part of the range. A northeast-trending positive magnetic anomaly, which is parallel to foliation trends reported in Precambrian rocks, extends from the Park Range across the North Park basin to the Medicine Bow Range. On the basis of this anomaly, the high seismic velocity of the Precambrian basement, and computed profiles fitted to the gravity and magnetic data, we infer that much of the basin is underlain by high-density metamorphic rock. As shown by gravity data, the deepest part of the basin is 2.7 km below sea level, resulting in a maximum relief of 6.7 km on the basement, relative to the Medicine Bow Range. A 25-mgal negative gravity anomaly and a zone of negative magnetic anomalies outline a large granitic intrusion in the Park Range, which probably extends northeast beneath the North Park basin and connects with granitic rocks in the Medicine Bow Range.

Chronostratigraphic relations of neogene formations of the Great Hungarian Plain based on interpretation of seismic and paleomagnetic data

This cooperative study by Hungarian and American geologists and geophysicists, demonstrates the power of the combination of modern paleomagnetic techniques and seismic profiling in working out the stratigraphy of a basin, with only limited paleontologic and isotopic control. Indirect evidence suggests that lake levels in the Pannonian inland sea (a remnant of the Paratethys), although isolated from the worlds oceans, were affected by eustatic sea-level changes. Four hiatuses identified by seismic profiles near the northern margin of the Pannonian Basin and inferred to represent non-deposition between 11.5 and 10.5, 7.9 and 7.6, 6.8 and 5.7, 5.4 and 4.6 million years ago. Comparing these hiatuses with the eustatic sea level change curves the accordance is close and systematic. This indicates that the sea level of the Pannonian Inland Sea that became gradually isolated from the world oceans fluctuated in the same phase as global sea level. The hiatus between 6.8 and 5.7 Ma is tentatively correlated with the Messinian global stage during which time evaporite deposition in the Mediterranean was widespread--the so-called Messinian salinity crisis.

Petroleum Geochemistry and Geology of Cenozoic and Mesozoic Sedimentary Rocks from Georges Bank Basin: ABSTRACT

On the basis of petroleum geochemical studies of the COST G-1 and G-2 well samples in the George Bank Basin, the Tertiary and Cretaceous sections between depths of 0 and 6,000 ft (1,829 m) are not believed to be prospective for oil or gas because of the thermally immature character of the lignitic and woody type of kerogens and extractable organic matter. The total organic carbon and extractable hydrocarbons of the Jurassic sedimentary rocks below 6,000 ft (1,829 m) indicate that these are poor to fair source rocks. Compositions of the C1 to C7 light and gasoline-range hydrocarbons, temperatures of maximum pyrolysis (TS2)°C, carbon preference indices (CPI), and thermal alteration indices (TAI) suggest that the shallowest thermally mature rocks are between 16,000 and 18,000 ft (4,877 and 5,486 m) in the G-2 well. High vitrinite reflectance (Ro) percentages may reflect oxidation and recycled organic matter. Estimates of the depth of thermal maturity based on measurements of the present geothermal gradient at the G-1 (1.2°F/100 ft) and G-2 (1.5°F/100 ft) wells are inconsistent with the depth of thermal maturity indicated by temperature-sensitive geochemical characteristics. The results of this study suggest that further consideration should be given to: (1) a reassessment of thermal-maturation processes in sediments associated with rift basins on passive margins; (2) the possibility that thermal maturation of the organic matter in the basin may have been influenced by a variable heat-flow history; (3) the location of the heat source, which may have been farther east of the present continental shelf than is presently thought. End_of_Article - Last_Page 1667------------

Petroleum Geology Of Baltimore Canyon Trough

Significant shows of natural gas have been reported from 3 out of 14 completed wildcat wells. The combined initial flow rates were about 2 million m/sup 3/ per day (68.5 million ft/sup 3/ per day) from Upper Jurassic sandstone beds at depths of about 4000 m. In the Baltimore Canyon trough, analysis indicates that shelf sediments have a relatively low potential for the generation of liquid hydrocarbons, but indications are favorable that natural gas could have been generated below a depth of 3000 m. However, sandstone porosities deteriorate below 3000 m, and at these depths, the Jurassic sandstone beds may lack lateral continuity. 28 refs.

Geologic Setting and Oil and Gas Potential of Eastern United States Continental Margin North of Cape Hatteras: ABSTRACT

Two sedimentary basins, the Georges Bank basin off New England and the Baltimore Canyon Trough off the Middle Atlantic states, have been examined for geologic setting and hydrocarbon potential. Georges Bank basin, a complex-shaped trough, is on a block-faulted basement of igneous, metamorphic, and sedimentary rocks. The deepest part of the basin (deeper than 8 km) and the oldest sediments are restricted to south-central Georges Bank. Toward the northeast and southwest, the sedimentary section thins to less than 2 km over the Yarmouth Arch-LeHave platform and the Long Island platform. The only deep wells in the area are the COST G-1 and G-2; data from these wells will not be released until 60 days after the first oil and gas lease sale. Seismic correlation with the Shell M hawk B-93 well on the Scotian Shelf indicates that most of the sedimentary rocks in the Georges Bank basin are Jurassic and older. Jurassic sandstone and limestone units serve as potential reservoir rocks. Potential hydrocarbon traps may occur on structural highs associated with draping of Jurassic and basal Lower Cretaceous strata over basement blocks. The Baltimore Canyon Trough is an elongated northeast-trending basin that contains at least 14 km of Jurassic and younger marine and nonmarine sedimentary rocks. Lithologic and stratigraphic data from the COST B-2 and B-3 wells indicate that Lower Cretaceous and Jurassic rocks are predominantly nonmarine to shallow-marine sandstone and shale. Analyses of organic carbon and identification of low thermal maturity suggest that gas rather than oil will be produced. Nineteen wildcat wells have been completed; three are significant natural-gas discoveries. The largest discovery is probably associated with a large rollover trap on the downthrown block of a Cretaceous and Jurassic growth fault. Potential hydrocarbon traps in carbonate rocks beneath the present continental slope have not yet b en explored. End_of_Article - Last_Page 746------------