Below Sea Level Research Articles

Observation and research of deep underground multi-physical fields—Huainan −848 m deep experiment

Compared with the surface, the deep environment has the advantages of allowing “super-quiet and ultra-clean”-geophysical field observation with low vibration noise and little electromagnetic interference, which are conducive to therealization of long-term and high-precision observation of multi-physical fields, thus enabling the solution of a series of geoscience problems. In the Panyidong Coal Mine, where there are extensive underground tunnels at the depth of 848 m belowsea level, we carried out the first deep-underground geophysical observations, including radioactivity, gravity, magnetic, magne-totelluric, background vibration and six-component seismic observations. We concluded from these measurements that (1) the background of deep subsurface gravity noise in the long-period frequency band less than 2 Hz is nearly two orders ofmagnitude weaker than that in the surface observation environment; (2) the underground electric field is obviously weaker thanthe surface electric field, and the relatively high frequency of the underground field, greater than 1 Hz, is more than two orders of magnitude weaker than that of the surface electric field; the east-west magnetic field underground is approximately the same asthat at the surface; the relatively high-frequency north-south magnetic field underground, below 10 Hz, is at least one order ofmagnitude lower than that at the surface, showing that the underground has a clean electromagnetic environment; (3) in additionto the high-frequency and single-frequency noises introduced by underground human activities, the deep underground spacehas a sig-nificantly lower background vibration noise than the surface, which is very beneficial to the detection of weakearthquake and gravity signals; and (4) the underground roadway support system built with ferromagnetic material interferesthe geomagnetic field. We also found that for deep observation in the “ultra-quiet and ultra-clean” environment, the existinggeophysical equipment and observation technology have problems of poor adaptability and insufficient precision as well asdata cleaning problems, such as the effective separation of the signal and noise of deep observation data. It is also urgent tointerpret and comprehensively utilize these high-precision multi-physics observation data.

Volcanic seismicity beneath Chuginadak Island, Alaska (Cleveland and Tana volcanoes): Implications for magma dynamics and eruption forecasting

Cleveland and Tana are remote volcanoes located in the central Aleutian volcanic arc on the eastern end of the Islands of Four Mountains (IFM). The persistently active Mount Cleveland volcano, on the western side of Chuginadak Island, is surrounded by several closely spaced Quaternary volcanic centers including Carlisle, Herbert, Kagamil, Tana, and Uliaga, and numerous small satellite vents on Chiginadak between Cleveland and Tana. The Alaska Volcano Observatory (AVO) installed two permanent broadband seismometers on Chuginadak Island in 2014, and we operated a temporary broadband network focused on the western side of the island in 2015–2016. Collectively, these stations provided the first seismic observations of this frequently active volcano and the surrounding Holocene-aged volcanic vents. During the study period (July 2014–January 2019), eruptive activity at Cleveland was characterized by small explosions separated by periods of lava effusion that formed small domes in the volcano's summit crater. We characterize seismicity beneath Chuginadak Island through automated analysis of event waveform frequency content, development of a one-dimensional P-wave velocity model, calculation of earthquake hypocenters, magnitudes, focal mechanisms, and identification of earthquake families. This analysis reveals the full range of seismic event types expected in a highly active volcanic environment and includes Volcano-Tectonic (VT) earthquakes, Long-Period (LP) events, and explosion signals. LP events appear to cluster at shallow depth beneath the active crater of Mount Cleveland and almost all of the explosions occur without identifiable short-term (hours to days) seismic precursors. VT earthquakes beneath Mount Cleveland occur at depths of 2 to 8 km below sea level (BSL) and range in magnitude from −0.2 to 1.8. VT focal mechanisms have horizontal P-axes that align with the regional axis of maximum stress. These observations, and a relatively slow one-dimensional seismic velocity model, are consistent with a shallow body of magma that is fed through a deeper conduit system. The time-history of VT earthquakes and shallow LP events suggest their occurrence may track the transfer of magma and fluids from the mid-crust to the shallow portions of the conduit system and may provide a means to anticipate future explosions and periods of dome growth. VT hypocenters also extend ~7 km northeast of Cleveland's summit at depths of 5 to 10 km BSL, under a group of Holocene-aged vents between Mount Cleveland and Tana. These earthquakes have vertically-oriented P-axes and a greater percentage occur in families. These observations, combined with observations of vent orientation and morphology and gas flux, suggest the area between Cleveland and Tana represents a zone of complicated volcano-tectonic interaction, similar to calderas elsewhere in the Aleutian arc. The presence of a larger volcanic system in the eastern IFM could influence magmatism and account for the multiple closely spaced volcanic centers in this region.

Seismic geomorphology and overpressure variation in the shallow-water-flow-prone sand units in the north-central Gulf of Mexico

The north-central Gulf of Mexico area received rapid deposition of a basin-floor fan system consisting of interbedded muds, silts, and sandy turbidite deposits during the Pleistocene. Overpressure occurs at shallow depths when burial rates exceed the dewatering rates of sediment pore fluids. Two stratigraphic sequences in the region contain significant overpressure with elevated shallow-water flow risk within these units. We have used publicly available seismic and well data to identify the geomorphology and overpressure variation of these units. The previously described “Blue Unit” and its lateral extent, thickness, depth below sea level (BSL), and overpressure gradient have been revised. The Blue Unit extends from the northern portion of the Mississippi Canyon (MC) protraction area to as far south as the Atwater Valley (AT) protraction area. For the first time, the Green Unit’s lateral extent, thickness, depth BSL, and pore pressure are defined. The “Green Unit” was found to extend further south than the Blue Unit into the AT protraction area and further east in the Desoto Canyon protraction area. The tops of both units are highly incised by postdepositional erosional systems, whereas the base of each unit is well preserved. The top of the Blue Unit below the mud line (BML) varies from <70 m (<230 ft) in the north to as deep as 701 m (2300 ft) in the south, whereas the top of the Green Unit is as shallow as 300 m (985 ft) in the north to 901 m (2956 ft) in the south. Overpressure in the MC area has been reported just BML. The pore pressure gradient ranges from 0.47 to 0.52 psi/ft at the base of the Blue Unit and increases to 0.60 psi/ft within the Green Unit.

Magmatic\u2013hydrothermal system of Aso Volcano, Japan, inferred from electrical resistivity structures

We investigated the magmatic–hydrothermal system of Aso Volcano, Japan, using broadband magnetotelluric (MT) data. To establish the nature of the shallow crust, a previous resistivity model based on data from 100 measurement sites in and around Aso volcano was revised using data from 9 additional sites near Naka-dake crater, which is located in the central part of the volcano. The components of MT impedance and the tipper vector were used to obtain the resistivity structure by three-dimensional inversion. The resistivity structure shows a subvertical low-resistivity (< 1 Ωm) column-shaped body beneath Naka-dake crater that extends from − 600 m to 10 km below sea level (BSL) and dips steeply to the north-northeast. The position of the upper part of the column is displaced eastward compared with the previous model and does not overlap the position of the presumed magma reservoir inferred previously from seismic and geodetic observations underneath the western side of Naka-dake crater at a depth of 5 km. We interpret this low-resistivity column to be a magmatic–hydrothermal system composed of brine and magma that were transported to Naka-dake crater from the main deep-seated magma reservoir. A horizontal low-resistivity (< 10 Ωm) layer occurs beneath post-caldera cones at the depths of 0–2 km BSL, and this layer extends laterally from the upper part of the low-resistivity column. We interpret this low-resistivity layer as representing a shallow hydrothermal system that has developed around the central column-shaped magmatic–hydrothermal system.

Influence of Climatic Phenomena on Sedimentation and Increase of Lake Enriquillo in Dominican Republic, 1900-2014

From 2003 to 2013, Enriquillo Lake, Dominican Republic, showed an increase in water level of 13 meters, with its level changing from 42 to 29 meters below sea level (BSL). In 2003, its maximum depth was approximately 24 meters; in 2013, it was 37.1 meters. The water surface coverage increased from 194.9 to 391 km2, flooding agricultural areas and leaving hundreds of people homeless. In addition, salinity increased from 105 ‰ to 23.4 ‰, a decrease of 4.47 times comparable to the volume change, which increased 4.48 times according to topographic and bathymetric maps. These changes began with Tropical Storm Odette and occurred progressively through 15 additional tropical cyclones (TC) or hurricanes that struck Lake Enriquillo between 2003 and 2011. Other climatic phenomena that have great influence on the Caribbean climate include the following: North Atlantic Oscillation (NAO), Southern El Niño Oscillation (ENSO), the trade winds, North Atlantic Warm Pool, Multidecadal Oscillation, the anticyclone Caribbean Regulator Climate Centers (CRCCs), and the Bermuda-Azores High (Mendez-Tejeda et al. 2017). The sediment records deposited by the runoff from these phenomena were analyzed. These proxy records were used to determine the decadal fluctuations related to cyclonic systems and the resultant effect upon increasing the level of Lake Enriquillo (LE). The accumulation rate (TAS) was estimated by means of 210 (210Pb) lead radiochronology, elemental composition, and mineral and ignition loss. These results were correlated with the extreme climatic events that affected the Lake Enriquillo Basin (LEB) from 1900 and 2011.

Physicochemical analysis of Lake Enriquillo in Dominican Republic

The water level in Lake Enriquillo in the Dominican Republic (DR) has increased by an unprecedented amount over the last decade, inundating thousands of acres of farmland and more than a dozen villages and inflicting adverse social, environmental, and economic impacts on the nation. The Lake Enriquillo Basin (LEB), located near the towns of Independencia and Bahoruco in the southwest region of the DR, is bordered to the north by the Sierra de Neyba mountain range and to the south by the Sierra de Bahoruco mountain range. Lake Enriquillo is hypersaline and endorheic, with a water level that normally fluctuates between 40–50 m below sea level (BSL). However, since 2000, both Lake Enriquillo and Lake Azuei (Sumastre) in Haiti have experienced alarming water level increases, with Lake Enriquillo’s water level rising to 17.2 m BSL This article explores the changes in some of the important physicochemical parameters—such as salinity, dissolved oxygen, temperature, and pH—that have occurred in the period from 1977 to 2012. The highest salinity value occurred in 2012, at a value 103.1‰, with an average increase of 27.06‰, coinciding with the lake level of 17.2 m BSL. Changes in these parameters have caused damage to the ecosystem and the lives of people and species that inhabit the region.

Feldspar dissolution, authigenic clays, and quartz cements in open and closed sandstone geochemical systems during diagenesis: Typical examples from two sags in Bohai Bay Basin, East China

Feldspar dissolution and precipitation of clays and quartz cements are important diagenetic reactions affecting reservoir quality evolution in sandstones with detrital feldspars. We examined two sets of sandstone reservoirs to determine whether the sandstone diagenetic systems were open or closed to the mass transfer of products from feldspar dissolution and its impact on reservoir quality. One of the reservoirs is the Eocene fan delta sandstone buried 2.5–4.0 km (1.5–2.5 mi) below sea level (BSL) in the Gaoliu (GL) area of the Nanpu sag, and the other is the Eocene subaqueous fan sandstone buried 1.5–4.5 km (1–2.8 mi) BSL in the Shengtuo (ST) area of the Dongying sag. Both sandstones consist mainly of lithic arkoses and feldspathic litharenites, and have secondary porosity formed by dissolution of feldspars. In the GL sandstones, the absolute amounts of authigenic clays and quartz cements (generally 125°C [257°F]). The low abundance of authigenic clays and quartz cements, and low pore-water salinity indicate that much of the , , and released from leached K-feldspars were exported from the GL sandstone system. And the extensive feldspar dissolution enhanced much porosity and permeability. In contrast, the ST sandstones with secondary pores formed by feldspar dissolution generally contain authigenic clays (kaolinite and illite) and quartz cements with almost identical volume of secondary pores. Kaolinite dominates in the ST sandstones at shallower depth (3.1 km [2 mi] BSL) where temperature exceeds 125°C (257°F). The presence of abundant clays and quartz cements indicates that and released from leached feldspars were retained in the ST sandstone system. The dominance of authigenic illite at greater depth indicates that sufficient should have been retained within the sandstones for occurrence of illitization of kaolinite and feldspars. Secondary porosity in thin sections can be up to 3%, but little porosity ( The diagenetic difference between the GL and the ST sandstones can be interpreted by assessing pore-water evolution in these two areas. The current pore waters with low salinity and negative hydrogen isotopic compositions in the GL sandstone system indicate the significant impact of meteoric water, whereas the current pore waters with high salinity and the paleofluids with positive oxygen isotopic compositions in the ST sandstone system indicate little trace of meteoric water. Access of meteoric freshwater to the GL area probably occurred during the late Oligocene to Neogene through widely developed faults in the Paleogene and Neogene strata. The low-salinity water could have been responsible for flushing of solutes derived from feldspar dissolution. As such, diagenesis in the GL sandstones is considered to have occurred in an open geochemical system, whereas with limited faults and high water salinity, the ST sandstones acted as a closed geochemical system where precipitation of kaolinite, illite, and quartz cements occurred following dissolution of feldspars.

제주도 우도 화산섬의 서브알칼리 현무암의 지화학적 특징에 대하여

Korea Institute of Geoscience and Mineral ResourcesUdo is the site of a short-lived monogenetic submarine volcanic vent which builts basaltic clastic deposits belowsea level, by early Surtseyan-type explosive eruption and later effusive lava eruptions. Mineral assemblage, majorand trace element chemistry indicate that the Someori sub-alkaline lava flows were related to the WPTB(withinplate tholeiitic basalt) rather than IAT(island-arc tholeiite) and that the geotectonic regime of Korean Peninsula wentaway from the subduction zone after the Later Miocene.

Dynamic deformation of Seguam Island, Alaska, 1992–2008, from multi-interferogram InSAR processing

We generated a time-series of ERS-1/2 and ENVISAT interferometric synthetic aperture radar (InSAR) images to study ground surface deformation at Seguam Island from 1992 to 2008. We used the small baseline subset (SBAS) technique to reduce artifacts associated with baseline uncertainties and atmospheric delay anomalies, and processed images from two adjacent tracks to validate our results. Seguam Island comprises the remnants of two late Quaternary calderas, one in the western caldera of the island and one in the eastern part of the island. The western caldera subsided at a constant rate of ~1.6cm/yr throughout the study period, while the eastern caldera experienced alternating periods of subsidence and uplift: ~5cm/year uplift during January 1993–October 1993 (stage 1), ~1.6cm/year subsidence during October 1993–November 1998 (stage 2), ~2.0cm/year uplift during November 1998–September 2000 (stage 3), ~1.4cm/year subsidence during September 2000–November 2005 (stage 4), and ~0.8cm/year uplift during November 2005–July 2007 (stage 5). Source modeling indicates a deflationary source less than 2km below sea level (BSL) beneath the western caldera and two sources beneath the eastern caldera: an inflationary source 2.5–6.0km BSL and a deflationary source less than 2km BSL. We suggest that uplift of the eastern caldera is driven by episodic intrusions of basaltic magma into a poroelastic reservoir 2.5–6.0km BSL beneath the caldera. Cooling and degassing of the reservoir between intrusions result in steady subsidence of the overlying surface. Although we found no evidence of magma intrusion beneath the western caldera during the study period, it is the site (Pyre Peak) of all historical eruptions on the island and therefore cooling and degassing of intrusions presumably contributes to subsidence there as well. Another likely subsidence mechanism in the western caldera is thermoelastic contraction of lava flows emplaced near Pyre Peak during several historical eruptions, most recently in 1977 and 1992–93.

Multiple causes for non-eruptive seismic swarms at Mt. Martin, Katmai Volcanic Cluster, Alaska (2004–2008)

In January 2006, the Alaska Volcano Observatory recorded a non-eruptive swarm of over 778 volcano-tectonic (VT) earthquakes in the Katmai Volcanic Cluster (KVC), Alaska. The swarm earthquakes have estimated hypocentral depths of 0–3km below sea level (BSL) immediately below the summit of Mt. Martin volcano. In an effort to determine the cause of this swarm, we calculated 134 double-couple fault plane solutions (FPS) for a pre-swarm period (Jan 2004–Dec 2005), the swarm period (January 2006), and a post-swarm period (Feb 2006–Dec 2008), and examined temporal changes in FPS orientations using directional statistical analysis. Statistical tests of uniformity indicate that FPS orientations were heterogeneous during the pre-swarm period, and homogeneous during the 2006 swarm. 3D tests on P- and T-axis orientation data also indicate that FPS homogeneity may have persisted during the post-swarm period. Best-fit axial Von Mises distributions indicate a strongly preferred NNE–SSW P-axis azimuth for swarm FPS, and a NW–SE mean P-axis azimuth for the pre- and post-swarm period FPS. The NW–SE background P-axis azimuth is approximately parallel to regional maximum compressive stress along the northern segment of the Aleutian arc. Thus, we find evidence for a temporary ~90° reorientation of FPS P-axes during the 2006 swarm, and suggest that the change in FPS orientation may be indicative of intrusion of a shallow, small-volume magma- or fluid-filled dike beneath Mt. Martin in early 2006. In contrast, FPS for VT earthquakes comprising minor swarms in 2007 and 2008 indicate only normal faulting, suggesting a fundamentally different causative process from the major 2006 swarm. Our results demonstrate that the VT earthquake swarms which are a common occurrence in the KVC may be caused by several processes, including magma intrusion, tectonic activity, and hydrothermal fluid circulation; and highlight the need for increased monitoring and analysis during future periods of geophysical unrest.

Seismic and geodetic investigation of the 1996-1998 earthquake swarm at Strandline Lake, Alaska

SUMMARY Microearthquake swarms occur frequently in volcanic environments, but do not always culminate in an eruption. Such non-eruptive swarms may be caused by stresses induced by magma intrusion, hydrothermal fluid circulation or possibly other tectonic processes such as slow-slip earthquakes. The Strandline Lake region of southcentral Alaska, located 30 km northeast of Mt Spurr volcano, experienced an intense earthquake swarm between 1996 August and 1998 September. A total of 2507 earthquakes were recorded by the Alaska Volcano Observatory's permanent seismic network during the swarm period, with a maximum magnitude of ML 3.6. The cumulative seismic moment of the swarm was 1.2 × 1015 N m, equivalent to that of a single Mw 4.0 earthquake. Because of the swarm's distance from the nearest Holocene volcanic vent, seismic monitoring was minimal and gas emission and GPS data do not exist for the swarm period. However, combined waveforms from a dense seismic network on Mt Spurr and from several regional seismic stations allow reanalysis of a representative set of swarm and background earthquakes. Swarm hypocentres calculated using a newly formulated 1-D velocity model and station corrections indicate a roughly circular swarm volume with dimensions of approximately 5 km, centred below 10 km below sea level (BSL). Composite fault-plane solutions for swarm earthquakes indicate oblique strike-slip faulting with a northeast-trending P-axis orientation. In contrast, a composite fault-plane solution for background earthquakes indicates a fundamentally different, though poorly constrained, local stress field orientation. Interferometric Synthetic Aperture Radar images spanning the swarm period unambiguously show no evidence of surface deformation, but do not rule out subcentimetre-scale deformation during the swarm. While a shallow ( 10 km BSL) magma intrusion representing an intrusive or protovolcanic segment of the Aleutian arc.

Comparative characteristics of the 1997–1998 seismic swarms preceding the November 1998 eruption of Volcán de Colima, México

The recent eruption of Volcán de Colima, México, began on 20 November 1998 and was preceded by a 12-month period of seismic activity that included five swarms of microearthquakes. The swarm activity occurred mainly within the southern part of Colima Volcanic Complex. We located about 600 events of magnitude M w from −0.5 to 2.7 within a 50 km 2 area including the active crater of Volcán de Colima and the region between it and the Pleistocene volcano Nevado de Colima 5.5 km to the north. The majority of hypocenters within this area did not exceed 5 km depth below sea level (BSL) and were mainly distributed within the volcanic edifices of Volcán de Colima and Nevado de Colima. We investigated the spatial distribution and spectral properties of events that occurred during three main seismic swarms: in November–December 1997, June–July 1998 and October–November 1998. We identified a temporal change in the depths of events from November–December 1997 and June–July 1998 (from 4 km BSL to 4 km above sea level (ASL)) to October–November 1998 (from 0 to 4 km ASL). Two groups of events with mean P-wave corner frequencies of 5 Hz (LF) and 10 Hz (HF) were distinguished. HF events dominated during the first swarm whereas the LF events were dominant during the June–July 1998 swarm. During October–November 1998, the numbers of HF and LF events were comparable, but very low-frequency low-stress drop events disappeared two days before the eruption. We consider the LF events to have resulted from fracturing along two systems of local faults, whereas the HF events could have been produced by dike movement along these same faults. We propose a model for construction of a magma passageway by the opening of two systems of local faults triggered by magma intrusion.

Case Study of Two Horizontal Wells for Improved Oil Recovery in New Zealand

Summary Two horizontal wells were drilled in the McKee reservoir to improve oilrecovery by repressing the coning problem experienced in conventional wells. Steps taken to facilitate an optimum completion in the geologically complexreservoir included petrophysical logging to identify fractured zones, cementingthe liner to provide isolation, and selective perforating. Disappointing earlygas breakthrough resulted in lower-than-expected oil recoveries. Introduction Today the application and drilling of horizontal wells is common. Because ofreduced costs and risks associated with horizontal drilling, hundreds of thesewells have been drilled successfully all over the world. The main benefits ofhorizontal wells, increased productivity and improved sweep efficiency, havebeen realized in many oil fields. Petrocorp Exploration Ltd. investigated thepossibility of drilling horizontal wells in the northern area of the McKeefield, which is characterized by a large gas cap overlying a thin oil column. Conventional wells in this area are plagued by high-GOR production and low oilrecovery. production and low oil recovery. This paper discusses the reservoirengineering aspects of the two horizontal wells drilled in the McKee field:justification and expectation,drilling and completion method, andperformance and production results. performance and production results. Field History The McKee field is located onshore Taranaki, New Zealand, some 20 miles eastof New Plymouth. The field is wholly owned and operated by Petrocorp. The fieldwas discovered in Sept. 1979 by the exploration Well McKee-1, and commercialproduction began in Oct. 1981. During 1980–89, the McKee field was developedwith 21 conventional wells from 11 surface locations producing to a centralproduction station. Before production started on the first horizontal well(Jan. 1990), the McKee field produced 10,600 BOPD and 9.0 MMscf/D gas, with a 5% water cut. Individual wells produced at rates from 200 to 1,450 BOPD. Allwells are completed in a single zone with sets of perforations spaced withblank sections to allow possible selective perforations spaced with blanksections to allow possible selective plugging of zones at later dates. pluggingof zones at later dates. In all wells, 2 7/8 -in. tubing has been used withsidepocket mandrels installed in case gas lift valves are needed later forartificial lift. All wells are still producing under natural flow. McKee Field Description The McKee field has two separate gas caps and a common water/oil contact(WOC). The gas/oil contact (GOC) is lower in the northern part of the fieldthan in the south. This results in an oil column part of the field than in thesouth. This results in an oil column in the north that is only 190 ft thick, whereas in the south and central areas, the oil column is 761 ft thick. Thedifferent GOC's arise from a saddle. The WOC was established at 7,100 ft truevertical depth subsea (TVDSS). The oil initially in place is estimated to havebeen some 106 MMSTB, and the gas initially in place is estimated at 183 Bscf. The accumulation occurs in the sandstone McKee formation around 6,562 ft belowsea level in one of several overthrust structures found in the Upper Eocene/Oligocene marginal marine sediments of eastern onshore Taranaki. Itbelongs to the Kapuni group and represents the last of a series of coastaltransgressions. The reservoir is sealed by the overlying marine claystone ofthe Turi formation and by the overthrust fault that juxtaposes the reservoirwith the Miocene marine claystone of the Mohakatino and Mahoenui formations. The McKee sand is, on average, some 230 ft thick. Although it has a homogeneousappearance superficially, it is heterogeneous on a larger scale. Thisheterogeneity results from the presence of subtle variations in grain size, thin calcareous-cemented beds, small-scale healed faults and fractures, andindistinct sedimentary structures. Reservoir quality suffers from significantdegradation of primary reservoir properties resulting from diageneticprocesses. processes. In the northern half of the field, the sand isinterbedded with a band of shales and thin coal beds that gradually thickentoward the north. This feature, known as the Intra McKee shale. divides the McKee formation into Sands A and B. Sand B averages 39 ft thick and is, onaverage, of poorer quality than Sand A. Most effective permeabilities to oilmeasured from well tests range from 15 to 20 md.

Development of a Mature Giant Offshore Oil Field

Summary The Teak field, a mature oil field offshore southeast Trinidad, has produced266 million bbl oil since 1972. Production is from Pliocene sandstonereservoirs in a Pliocene sandstone reservoirs in a complexly faulted anticline. This paper summarizes the history and geology of Teak field, describes itsoil-producing reservoirs, and discusses the mature-field development strategyused since 1988, including case histories of two 1989 developmentprospects. Introduction The Teak field, the fourth largest oil field in Trinidad, is located 25miles off the south-east coast of the island in 190 ft of water (Fig. 1). Discovered by Amoco Production Co. in 1969, Teak has produced 266 million bblof oil and 1.1 trillion scf of gas from 1972 through 1990. Teak is a mature oilfield with significant depletion after its first 19 years of production. Despite a steep natural decline rate of 25%/yr, the field has respondedfavorably to renewed development drilling since 1989. A steady downward trendin production rate reversed, and the field achieved its highest rate in almost5 years in July 1990. A primary factor in the reversal of the decline rate wasa deliberate multidisciplinary team approach to reservoir development. Geology Structural Setting. The Teak field is located in a regionalcompressional-wrench terrain about 40 miles south of the current Caribbean and South American plate boundary along the El Pilar fault (Fig. 1). The fieldproduces hydrocarbons from Pliocene sandstones from 4,000 to 14,000 ft belowsea level. Hydrocarbons are trapped in a steep anticlinal structure along aneast/west-trending compressional ridge bisected by a large fault (Fig. 2). Thisfault dips east-northeast at 35 to 45 degrees, with vertical offset of 2,000 to3,000 ft, splitting the field into two separate hydrocarbon accumulations: oiltrapped to the west on the upthrown side of the fault and gas trapped to theeast on the downthrown side of the fault. In addition to normal offset, Fault Fexhibits left-lateral wrenching, indicated by displacement of the downthrownstructural crest about 1 mile north of the upthrown crest. This paper focusesprimarily on the upthrown oil reservoirs, which are arranged as stackedthree-way closures behind the major sealing Fault F and are between 4,000 and12,000 ft subsea (ss) (Fig. 3), informally called the Teak A-C-E field. The Teak structure is characterized by fairly steep bed dips that average 10degrees and steepen locally to more than 30 degrees. Bed dips are locallyvariable in both direction and magnitude. The structural geometry iscomplicated by myriad cross-cutting normal faults that segment the field intonumerous compartments. The oil accumulation up-thrown to Fault F is separatedinto two major fault blocks by Fault F (800 to 1,500 ft vertical offset) andseveral minor fault blocks. About 90% of the Teak field oil produced to datehas been trapped upthrown to both Faults F and F2, although deeper reservoirshave been (discovered and produced between these faults. Stratigraphy and Petrology. Teak oil reservoirs span an 8,700-ft-thickstratigraphic section in the Pliocene that contains approximately 60 % sand and 40% shale. Teak sands are typically very fine-grained (from 0.06 to 0.12 mm ingrain size) and extremely well-sorted, and average more than 90% quartz withlittle or no visible cement. Porosities average 27 to 33%. Teak shales Porosities average 27 to 33%. Teak shales exhibit a variable resistivity andgamma ray log response, unlike clean shales typical of the U.S. Gulf of Mexico. Shale lithologies vary from argillaceous shales to arenaceous mudstones, andtextures vary from soft to well-indurated and phyllitic. Oil-producing sandsexhibit no appreciable stratigraphic thickness variation across theoil-productive area. The Pliocene producing section is 10 to 20% thicker on thedownthrown side of Fault F, but individual sands maintain an excellentcorrelation across the field. The environment of deposition for the Pliocenesands is probably a broad, shallow Pliocene sands is probably a broad, shallowmarine shelf proximal to the Orinoco delta, where strong, steady coastalcurrents provided a uniform energy regime. Oil Reservoirs Reservoir and Fluid Parameters. High-quality crude oil, 27 to 33 degrees API, has been produced from 17 different sandstone reservoirs (Fig. 3 and Table1). Individual sands vary from 10 to 440 ft in thickness. Sand porosities rangefrom 18 to 33%, decreasing slightly with depth. Initial water saturationsaverage 25%, with residual oil saturation of 20%. Hydrocarbon columns rangefrom 170 to 900 ft of oil with thin gas caps. April 1991 P. 392

Stratigraphy and Radiocarbon Chronology of a Subaerially Exposed Holocene Coral Reef, Dominican Republic

A subaerially exposed coral reef records a period of marine conditions following earliest Holocene transgression into the technically active, subsea level Enriquillo Valley, Dominican Republic. Field mapping, detailed leveling of reef elevations, and I4C dating of fossil corals and mollusks reveal the following chronology of reef development in an unusual geologic and tectonic environment: (1) earliest Holocene ($$9760 \pm 100$$ yr B.P.) marine transgression and growth of oysters on an alluvial fan surface approximately 33 m below sea level (BSL); (2) colonization of reef corals at approximately 34 m BSL on the oyster shell deposits ($$8990 \pm 60$$ yr B.P.); (3) middle Holocene development of a typical Caribbean fringing reef in a protected environment ($$6200 \pm 80$$ to $$4760 \pm 90$$ yr B.P.); (4) late Holocene sedimentation over the reef with fossil bivalves ($$5710 \pm 90$$ to $$2820 \pm 40$$ yr B.P.) that record a gradual change from marine to brackish water that accompanied growth of shoreline th...