Igneous Sills Research Articles

Structural and stratigraphic evolution of the Anza rift, Kenya

The Anza rift is a large, multi-phase continental rift basin that links the Lamu embayment of southern Kenya with the South Sudan rifts. Extension and deposition of syn-rift sediments are known to have commenced by the Neocomian. Aptian-Albian strata have, thus far, not been encountered during limited drilling campaigns and, in at least one well, are replaced by a significant unconformity. Widespread rifting occurred during the Cenomanian to Maastrichtian, and continued into the Early Tertiary. Marine waters appear to have reached the central Anza rift in the Cenomanian, and a second marine incursion may have occurred during the Campanian. As no wells have yet reached basement in the basinal deeps, the possibility exists that the Anza rift may have initiated in the Late Jurassic, in conjunction with extension to the south in the Lamu embayment and to the north in the Blue Nile rift of Sudan. Structural and stratigraphic evolution in the Anza rift followed a pattern that has now been inferred in several rift settings. Early phases of extension were accommodated by moderately dipping faults that produced large stratal rotations. Sedimentary environments were dominantly fluvial, with associated small lakes and dune fields. Volcanic activity is documented for the early Neocomian, but its extent is unknown. This initial style of deformation and sedimentation may have continued through several of the earliest pulses of rifting. By the Late Cretaceous, a new system of steeply dipping faults was established, that produced a deep basin without significant rotation of strata in the north, and only minor rotation in the south. This basin geometry favored the establishment of large, deep lakes, which occasionally were connected to the sea. The older basins were partly cannibalized during the sedimentary in-filling of these successor basins. Early Senonian volcanism was encountered in one well, and reflection seismic evidence suggests that one or more thick, regionally extensive igneous sills were intruded, probably during the Early Tertiary. The change in rift style from early, strongly rotational, shallow basins to late, non-rotational, deep basins has been observed in the southern Gulf of Suez/northern Red Sea, the Southwestern Turkana/northern Kenyan rift, and at Anza. It therefore takes place in rifts in variable tectonic settings, with a wide range of volcanic activity and, presumably, with different driving mechanisms. The shift in deposition in each case is away from early rift-bounding faults toward the half-graben flexural margins, further in-board to the upper structural plate. This suggests at least some component of regional simple shear in the deformation history of the rifted lithosphere, either via broad shear zones or at discrete detachment surfaces.

Interpretation of the magnetic anomaly over the Omaha Oil Field, Gallatin County, Illinois

A 40 nanoTesla (nT) magnetic anomaly identified in an aeromagnetic survey over southern Illinois contours as a localized magnetic high on the west flank of a regional magnetic low. This magnetic anomaly is generally coincident with the Omaha Oil Field in northwest Gallatin County, Illinois. It was initially assumed that cultural sources of steel associated with this oil field were the primary source of the magnetic feature; however, similar oil fields overflown by the survey do not exhibit magnetic anomalies in the data set. The Luther Rister et ux ♯1 well, drilled near the apex of the Omaha structural dome, encountered two zones of ultramafic intrusive rock containing 9.0 percent by volume magnetite. These intrusives were identified to be alnöites which are a class of mantle‐derived ultramafic rock that can be associated with the incipient stages of crustal rifting. A ground magnetic survey verified the presence of the anomaly, and provided detailed data for 3-D modeling of the source. Petrophysical evaluations, magnetic susceptibility measurements and thin section modal analysis were made on drill cuttings from the ultramafic intrusives encountered in the Luther Rister ♯1 well. These measurements were made to constrain the 3-D magnetic modeling by the petrophysical characteristics of the source. After removal of the regional magnetic field, the resulting 140 nT residual magnetic anomaly was successfully modeled using two ultramafic sills with an igneous feeder plug. The two igneous sills adequately account for the structural closure exhibited in the Omaha Oil Field and raise the interesting possibility of other hydrocarbon trapping structures generated by intrusives emplaced into the sedimentary section.

Dinosaur tracks in intrusive igneous rock

Well preserved brontosaur footprints are replicated as impressions at the top of a dacitic igneous sill that intruded the track‐rich Cretaceous Jindong Formation, Kyeongsang basin South Korea. Although an unusual occurrence, the example shows the potential of such intrusions for enhancing preservation of trace fossils in some cases

Rift architecture and sedimentology of the Phetchabun Intermontane Basin, central Thailand

The Phetchabun Basin, located onshore in central Thailand is one of at least 30 Tertiary intermontane basins identified in Thailand. The basin is a composite of several north-south trending half and full graben, which formed through transtensional dextral shear along the Mae Ping fault zone. Several conjugate strike-slip faults cut the basin and may have acted as the mechanism for formation of the individual graben. The grabens vary in depth from 2500 to 1100 m with the deepest being in the south and progressively shallowing northwards. Seismic cross sections across the basin reveal typical rift structuring. Central graben horsts, hanging wall anticlines and rotated fault blocks predominate within the graben, while wrench style structures are evident along inferred transfer zones. Sedimentation commenced in the Oligocene with syn-rift fluvial deposits and associated rift volcanics. An Oligocene to Mid-Miocene sedimentary sequence (Wichian Buri Group) of fluvial and lacustrine deposits followed. Diorite and diabase intrusives reflect periods of igneous activity during the Early and Late Middle Miocene. After the Mid-Miocene tectonic episode, lacustrine conditions were re-established over much of the basin and predominantly fine-grained sediments were deposited (Chaliang Lab Formation). Plio-Pleistocene sediments comprising lithic sands and shales were deposited in an oxidizing alluvial environment. Waxy oils and dry gas have been recovered from thin bedded sandstone reservoirs and igneous sills. Initial tests at Wichian Buri-1 indicated flow rates up to 500 BOPD from thin sands, while Bo Rang-1 tested gas at rates up to 5.5 MMCFG/D from an altered sill.

Depositional Environments and Geometry of Coal-bed Natural Gas Reservoirs in the Raton Basin, Colorado and New Mexico

Recognition of the depositional environments, occurrence, and geometry of Raton basin coals and coal-bearing rocks is essential to the reliable estimation of potential gas-in-place reserves and profitability, optimum placement and operation of production wells, and effective completion design for all Raton basin coalbed degasification projects. This knowledge is needed for two reasons: (1) coals of the Vermejo and Raton Formations, coalbed gas reservoirs in the Raton basin, are laterally discontinuous and separated by significant innerburden. (2) Vermejo and Raton Formation coals have been naturally coked or replaced by middle Tertiary igneous sills and dikes in many areas of the basin. Diagnostic criteria for facies recognition indicate the major environments of peat deposition included generally northeast-trending Vermejo interdistributary bays, northwest-trending Vermejo distributary-mouth bars, and northeast-trending Raton alluvial plains. Stratigraphic, sedimentologic, and coal petrographic data indicate Raton basin coals are vitrinite rich, composed of discrete coal and shale interbeds 2-10 ft thick at drilling depths of up to 2,500 ft, present in a stratigraphic interval at least 1,000 ft thick, that collectively form net thicknesses of 15 to 70 ft and dramatically thicken, thin, pinch out, or abut against sills or dikes in distances of less then 5,000 ft. The data basemore » for these conclusions is comprised of petrographic observations for approximately 200 coal crushed particle pellets, 54 measured sections, 294 paleocurrent measurements at 45 stations, geophysical logs for 50 wells, and observations of field depositional relationships at over 200 outcrops.« less

Habitat of Petroleum in the Paleozoic Basins of Brazil: A Look Back, a Comparison, a Look Forward

It took almost a century of intermittent petroleum exploration in Brazil's vast Paleozoic basins before continuous production of about 5000 bbl per day of light oil, together with gas, could be established in one of them. Proved volumes of oil equivalent in place so far discovered in Paleozoic reservoirs amount to only 0.6% of the domestic total, but hopes of substantial new finds are great. In the Rio Urucu field of Solimoes basin (Upper Amazon), hydrocarbons are being produced from Early Carboniferous Monte Alegre formation sandstones roofed by evaporites and carbonates of the Carboniferous Itaituba formation which were intruded by thick Juro-Triassic igneous sills and folded during a Cretaceous reactivation event into the present-day shape of gentle anticlinal trends controlled by reverse faulting. The widespread occurrence of igneous intrusions and extrusions in the Solimoes, Amazonas, Parnaiba, and Parana basins is a major problem met by explorationists throughout the years but can be faced with presently available technology, seismic and other. Further investigations in these four basins are needed in order to better define details of basin architecture, configuration of basement, location of the oil kitchens, main carrier beds, windows in the dyke walls through which hydrocarbons could find their waymore » to the traps, etc. Examples from the various basins, along with a comparison to some successful exploration cases in the USA, suggest that the habitat of other commercial oil and gas in the Paleozoic basins of Brazil may be localized in the deeper basinal positions - like in the Rio Urucu trend - but also in the shallow flank areas, where igneous rocks tend to be scarcer and exploration baroquely started.« less

The origin of fluvial valleys and early geologic history, Aeolis Quadrangle, Mars

In southern Aeolis Quadrangle in eastern Mars, parallel slope valleys, flat‐floored branching valleys, V‐shaped branching valleys, and flat‐floored straight canyons dissect the heavily cratered plateau sequence. Associated knife‐like ridges are interpreted as fissure eruption vents, and thin, dark, stratiform outcrops are interpreted as exhumed igneous sills or lava flows. Ridged lava plains are also common but are not themselves modified by fluvial processes. I mapped 56 asymmetric scarps or ridges that are probable thrust faults. These faults exhibit an orientation vector mean of N63°W±11° (95% confidence interval), and they transect the lava plains and the older plateau sequence units. By comparison, the vector mean for the 264 valleys mapped is N48°W±12°, with a larger dispersion about the mean. The similar orientations displayed by thrust fault and valley axes suggest that valley locations are partly controlled by preexisting thrust faults and related fracture systems. Most valleys are also arranged orthogonally to, and along the perimeter of, the ridged plains. A possible model for valley development is: (1) freshly outgassed water became entombed as frost, snow, and ice within the cratered terrains during heavy bombardment and the accompanying deposition of impact ejecta, volcanic ash, and eolian materials, (2) effusive volcanism and lava sill emplacement heated subsurface ice in the vicinity of the ridged plains, and faults and fractures provided zones of increased permeability for water transport to the surface, and (3) headward sapping at thermal springs, thermokarst subsidence, and limited downvalley fluid flows then carved and modified the valleys.

Seismic Expressions of Igneous Sills in San Juan Sag, South-Central Colorado: Humps or Bumps, Tricks or Traps?: ABSTRACT

Petroleum exploration in the San Juan sag of south-central Colorado has been an exciting but frustrating experience. Thick surface volcanics cause seismic data to be fair to poor quality and often make mapping of the stratigraphy difficult. Some of the strongest reflections are igneous sills intruded parallel to the bedding planes of prospective horizons. Sills are dense, high-velocity (13,000-18,000 ft/sec) layers which stand out as high-contrast reflections in the lower velocity Cretaceous shales and to a lesser extent in the similar velocity Dakota, Morrison, and Junction Creek sandstone sections. Almost every well drilled in the San Juan sag has had at least one sill present in the Cretaceous or Jurassic section. Sills vary in thickness from 6 to 1,036 ft, with most common thicknesses from 200 to 500 ft. On seismic data the strong reflections caused by the velocity contrast of the igneous rock make the sills stand out as amplitude anomalies. Since the sills are additive to the section, they can cause bumping or thickening which may appear as a structural bump. Several wells have been drilled on these apparent structures. Color displays of seismic amplitude and phase can help detect igneous sills. Ironically, the only productive well inmore » the San Juan sag to data has an igneous sill as its reservoir. The Del Norte field produced approximately 1,500 bbl of oil from fractured sill intruded into the Mancos Shale. The sill is 58-155 ft thick and is visible on seismic. Although rare, sills can be hydrocarbon traps as illustrated by the Dineh-bi-Keyah field of northeastern Arizona, which has produced more than 15 million bbl of oil.« less

Melt-enhanced deformation: A major tectonic process

Convergent tectonics between continental crustal blocks result in deep burial and anatexis of supracrustal rocks. Anatectic and/or mantle-derived melts combine to form a melt-weakened zone in the thickened lower crust. The accumulation of melt eventually leads to crustal failure along melt-lubricated shear zones. Rapid (>1 mm/yr vertical component) movements of large crustal blocks result. We refer to these crustal displacements as tectonic surges. The melt-lubricated shears are characterized by the close association of sheared country rocks with foliated or massive igneous sills and plutons. Rocks that formed at different crustal levels are juxtaposed across these shear zones. One result of surges with large lateral component of movement is metamorphic inversion with high-pressure and high-temperature metamorphic rocks structurally over lower pressure and temperature assemblages. Large and rapid vertical surges may displace crust containing abundant melt and may result in high T/P (including granulite facies) metamorphism and preservation of metamorphic textures caused by rapid decompression.

Washington Ranch Morrow Field: Case History in Frontier Exploration: ABSTRACT

The Washington Ranch Morrow gas field is located 35 mi (56 km) southwest of Carlsbad, New Mexico, on the western rim of the Delaware basin 6 mi (10 km) basin-ward from the Capitan reef front. Late Pennsylvanian orogenic forces created a structural development of such sharp magnitude that most exploration outlook condemned the area as too disturbed for commercial hydrocarbon entrapment. The northwest-southeast-trending fault on the northeast side of the field has a displacement of more than 2,000 ft (610 m), structural contours on the basal Morrow sandstone unit define 600 ft (183 m) of closure against the fault. The crest of the structure has lost 1,700 ft (518 m) of Wolfcampian and Upper Pennsylvanian section through erosion and nondeposition. Eight deep dry holes had been drilled within an 8 mi (13 km) radius of the drill site for the discovery well. Several of these wells had encountered a significant igneous sill only 400 ft (122 m) below the basal Morrow sandstone. The presence of this sill so close to the main potential reservoir rock further detracted from the prospect. Reefing development of two separate ages in Pennsylvanian sediments caused very rapid lateral facies changes which made well log correlations difficult. Frontier exploration philosophy and technique were employed to overcome the many negative considerations of the prospect. The discovery well was completed in June 1971 for a calculated absolute open flow potential of 54 MMCFGD natural, through Morrow perforations at 6,795 to 6,806 ft (2,071 to 2,074 m). While drilling was in progress gas flowed at the rate of 22 MMCFGD on a 2-hour and 10-minute drill-stem test from 6,791 to 6,860 ft (2,070 to 2,091 m). Major gas reserves were made available to the pipeline with the drilling of nine development wells. End_of_Article - Last_Page 766------------

On the mechanical interaction between a fluid-filled fracture and the earth's surface

The mechanical interaction between a fluid-filled fracture (e.g., hydraulic fracture joint, or igneous dike) and the earth's surface is analyzed using a two-dimensional elastic solution for a slit of arbitrary inclination buried beneath a horizontal free surface and subjected to an arbitrary pressure distribution. The solution is obtained by iteratively superimposing two fundamental sets of analytical solutions. For uniform internal pressure the slit behaves essentially as if it were in an infinite region if the depth-to-center is three times greater than the half-length. For shallower slits interaction with the free surface is pronounced: stresses and displacements near the slit differ by more than 10% from values for the deeply buried slit. The following changes are noted as the depth-to-center decreases: 1. (1) the mode I stress intensity factor increases for both ends of the slit, but more rapidly at the upper end; 2. (2) the mode II stress-intensity factor is significantly different from zero (except for vertical slits) suggesting propagation out of the original plane of the slit; 3. (3) displacements of the slit wall are asymmetric such that the slit gaps open more widely near the upper end. Similar changes are noted if fluid density creates a linear pressure gradient that is smaller than the lithostatic gradient. Under such conditions natural fractures should propagate preferentially upward toward the earth's surface requiring less pressure as they grow in length. If deformation near the surface is of interest, the model should account explicitly for the free surface. Stresses and displacements at the free surface are not approximated very well by values calculated along a line in an infinite region, even when the slit is far from the line. As depth-to-center of a shallow pressurized slit decreases, the following changes are noted: 1. (1) displacements of the free surface increase to the same order of magnitude as the displacements of the slit walls, 2. (2) tensile stresses of magnitude greater than the pressure in the slit are concentrated along the free surface. The relative surface displacements over a shallow vertical slit are downward over the slit and upward to both sides of this area. The tensile stress acting parallel to the free surface over a shallow vertical slit is concentrated in two maxima adjacent to a point of very low stress immediately over the slit. The solution is used to estimate the length-to-depth ratio at which igneous sills have gained sufficient leverage on overlying strata to bend these strata upward and form a laccolith. The pronounced mode II stress intensity associated with shallow horizontal slits explains the tendency for some sills to climb to higher stratigraphie horizons as they grow in length. The bimodal tensile stress concentration over shallow vertical slits correlates qualitatively with the distribution of cracks and normal faults which flank fissure eruptions on volcanoes. The solution may be used to analyze surface displacements and tilts over massive hydraulic fractures in oil fields and to understand the behavior of hydraulic fractures in granite quarries.

An approach to engineering geological investigations on the Island of Montreal

In Montreal, knowledge of extent of saturated sands and marine clays is essential before the construction period begins as many foundations require the excavation of these materials. Details of clay seams in limestones and decomposed igneous sills and dykes are needed, but the discovery of areas where there is an upper zone of 10 to 20 ft (3 to 6 m) of disturbed bedrock is of greater importance. Thus investigations should be concentrated in local problems.The maintenance of an up-to-date central inventory of all geological and geotechnical data together with information on the constructional difficulties and costs involved is desirable, but it appears only practicable to index work as a general rule.The review of a past investigation showed that in an area of simple geology the standard spacing of drillholes was appropriate and successfully defined bedrock morphology and elevations; however, elsewhere on the Island only chance or a greater density of investigation may reveal bad conditions.Investigations can only be optimum if the ‘users’, the structural designers, quantity estimators, construction supervisors as well as contractors are required to acknowledge formally the impact of the geological and geotechnical appraisals upon their planning and operations including the effects of conditions suspected to be present but not actually confirmed.

Mesozoic Geology of Svalbard: ABSTRACT

Mesozoic rocks are known from most of the major islands of Svalbard, namely Spitsbergen, Nordaustlandet, Barentsoya, Edgeoya, Kong Karls Land, Hopen, and Bjornoya. Sedimentary rocks range in age from Triassic (early Scythian) through Early Cretaceous (Albian), with facies mostly drab shale, siltstone, and sandstone--generally marine shale and continental sandstone--and comprise 2 lithostratigraphical units: Sassendalen Group (Griesbachian to Toarcian) and Adventdalen Group (Bathonian to mid-Albian). These units contrast markedly with the underlying Permian cherty carbonates, and not so obviously with the resistant overlying Tertiary coal measures. The marine strata are characterized by ammonites, bivalves, and saurians; the continental strata have plant beds, thin coal seams, some bivalves, and vertebrates. The succession and facies are very similar to those of Arctic Canada. The most conspicuous rocks in the older part of the sequence are the cliff-forming basic igneous sills and flows of latest Jurassic and/or Early Cretaceous age. The Mesozoic tectonic pattern followed a relatively stable late Paleozoic history with a marked change of facies but conformable strata. The maximum known thickness of Mesozoic strata is about 3 km. The first distinguishable disturbance (warping and faulting) accompanied basic igneous activity but with little change of sedimentary facies. The principal unconformity represents a hiatus which took place in late Albian to early? Paleocene time. There is local overstep of Tertiary rocks onto lowermost Triassic, but generally only the uppermost Albian members are missing. These minor disturbances may be related to movements that culminated in the West Spitsbergen Orogeny in early to mid-Tertiary time, and to the mainly Tertiary Arctic Ocean spreading. Svalbard was probably moved from subtr pical to temperate latitudes in Mesozoic time, only subsequently achieving arctic latitudes. Petroleum prospects in the Arctic must take Mesozoic rocks into account as providing source, cap, and reservoir rocks. End_of_Article - Last_Page 2484------------

Geochronology Applied to Exploration Problems: ABSTRACT

The present facilities for isotope age measurements at the Shell Development Company, Exploration and Production Research Division Laboratory, permit measurement by all of the well established age methods--potassium-argon, rubidium-strontium, uranium-lead, and thorium-lead. This gives us the advantage of a wide choice of minerals, and at least among the igneous and metamorphic rocks some age data can usually be obtained by at least one of the methods mentioned. Age measurements are often used to determine the age of igneous rocks encountered in wells. This enables the geologist to judge whether additional sedimentary reservoirs could be expected. In some places the age might suggest that true igneous has been reached and that no further sedimentary section can e reasonably expected. In other places, the presence of igneous sills and dikes, or volcanic layers is suggested, below which additional reservoirs could be expected. Samples of bottom-hole rocks from wells drilled prior to the general use of age determination methods have now been examined with the same objective. In general the results of these measurements on wells in different parts of the United States can be shown to fall in line with the suggested basement periods (G. R. Tilton and S. R. Hart, Science, vol. 140, p. 357, 1963). Frequently age determinations are used to determine time and influence of igneous activities in new exploration areas. In this case surface samples are used. This type of information helps to unravel the structural evolution of an area. This is important background for any study of the hydrocarbon potential of a new exploration province. The use of glauconite for potassium-argon and rubidium-strontium dating of sediments has been common for many years in oil exploration for obvious reasons. The use of the minerals in bentonite to date sediments is the subject of another paper in this symposium (R. E. Folinsbee, H. Baadsgaard, G. L. Cumming, and J. Nascimbene, Radiometric Dating of the Bearpaw Sea). The first dates on both biotite and zircon in bentonites were those measured at the Shell laboratory. Recently we have been measuring the age of detrital minerals in sediments as an aid in determining the source of conglomerates and micaceous sandstones, and this work is continuing. End_of_Article - Last_Page 524------------

The Petrology of a Teschenite Sill at Landywood, Staffordshire

AbstractBorehole cores have been obtained of a basic igneous sill at Landywood, near Cannock, Staffordshire. This intrusion consists mainly of microteschenite with a zone of teschenite in the top half of the sill. Later parts of the parental magma of this intrusion were dryer and crystallized to give crinanite and analcite olivine dolerite. These later parts were not intruded in sufficient quantities to spread through the whole of the sill and were restricted to the vicinity of the feeding pipe. Extensive secondary replacement of plagioclase by chlorite and of pyroxene by carbonate occurs in the teschenitic parts of the sill and is ascribed to the action of water and carbon dioxide derived from the magma.

Antarctica: Geology of the Ellsworth Mountains

Geologic reconnaissance indicates that the Ellsworth Mountains consist mainly of thousands of feet of folded, slightly metamorphosed, clastic sedimentary rocks of unknown age. Three major stratigraphic units are recognized, but only fragmentary fossils have been found. The folding is asymmetric, overturned, or recumbent; fold axes strike north, 10 degrees to 20 degrees west. Basic igneous sills occur in the northern Heritage Range.

The Igneous Rocks of Southwest Ecuador

The coast-belt of Ecuador, lying between 1 and 3 degrees south of the Equator, consists of Tertiary formations with which are associated igneous dikes and sills and occasional lava flows. This article deals entirely with the igneous rocks of the region, and an account is presented of the distribution, geological history, and the petrological details of the respective rock types. An attempt has been made to show that a petro-graphic province may occur in this region, and a correlation of the different rocks has been made which indicates that the intrusions bear a constant relation to the Tertiary formations with which they are associated.