Maturin Subbasin Research Articles

Study of magnetic contrasts applied to hydrocarbon exploration in the Maturín Sub-Basin (eastern Venezuela)

We have studied near-surface magnetic contrasts in nine oil wells from an oil prospective area in eastern Venezuela (compressive deformation front of the northeastern Maturin Sub-Basin). Samples are drill cuttings taken at intervals of about 15 m within approximately the first 1200 m of most of these wells. Rock magnetic experiments, electron paramagnetic resonance (EPR), extractable organic matter (EOM), X-ray diffraction and scanning electron microscopy (SEM) are used to discriminate between two anomalies in magnetic susceptibility (MS), related to either a reducing environment caused by the underlying reservoir (Type A), or to primary lithological contrasts (Type B). Contour maps of MS, S-ratio and organic-matter-free radical concentrations (OMFRC), for the A-like depth levels, show a major central zone of anomalous high values. This area is probably associated with either maximum accumulation of hydrocarbons in the reservoir or with their southerly migration from the northern petroleum source towards the deformation front. We argue that such a result could be used, for future exploration and production ventures in the region, as a preliminary characterization of the reservoir. We also show in a Day plot, that saturation remanence — saturation magnetization ratio (Mrs/Ms) and coercivity of remanence — coercive force ratio (Hcr/Hc) stand as additional criteria to discriminate between these two types of magnetic contrasts. In fact, for the area of study, hysteresis ratios show three distinct trends corresponding to samples from A-like, B-like and non-anomalous depth levels.

Sequence stratigraphic analysis of a semiregional depth‐migrated profile in eastern Venezuela

The onshore Eastern Venezuela Basin, the second‐richest hydrocarbon province of South America (trailing only the Maracaibo Basin), is a complex region where an intricate set of listric normal faults developed in the foreland basin of the Monagas folded belt during the Neogene. This paper will describe the application of a combination of geophysical and geologic techniques for stratigraphic interpretation of semiregional profile of the Maturin Subbasin.

Comparison of the Petroleum Systems of East Venezuela in Their Tectonostratigraphic Context: ABSTRACT

The Maturin and Guarico subbasins of East Venezuela record the transition from Cretaceous passive margin to Tertiary foreland basin with local post-orogenic transtensional basins. Petroleum is reservoired in several units ranging from Albian (El Cantil Formation) to Pliocene (Las Piedras Formation) age. Source rocks are principally in the Upper Cretaceous (Querecual Formation), and Miocene (Carapita Formation) in the Maturin subbasin and in the Upper Cretaceous (Tigre Formation) and Oligocene (Roblecito and La Pascua Formations) in the Guarico subbasin. An extensive well database has been used to address the distribution and provenance of hydrocarbons in the context of a tectonostratigraphic model for the evolution of the East Venezuela basin. Nine major plays have been described, comprising thirteen petroleum systems. The principal factors influencing the components of individual petroleum systems are as follows: (1) structural controls on Upper Cretaceous source rock distribution, relating to block faulting on the proto-Caribbean passive margin; (2) paleoenvironmental controls on source rock development within the Oligocene-Miocene foreland basin; and (3) timing of subsidence and maturation within the Oligocene-Upper Miocene foreland basin and the configuration of the associated fold and thrust belt, influencing long range and local migration routes (4) local development of Pliocene post-orogenic transtensional basins, influencingmore » hydrocarbon generation, migration and remigration north of the Pirital High.« less

Seismic Data in the Southern Maturin Sub-Basin in Eastern Venezuela Reveals the Structure and Strata I Patterns of the Lower Miocene Sediments: ABSTRACT

Seismic Data in the Southern Maturin Sub-Basin in Eastern Venezuela Reveals the Structure and Strata I Patterns of the Lower Miocene Sediments: ABSTRACT

Organic geochemistry of the Carapita Formation and terrestrial crude oils in the Maturin Subbasin, Eastern Venezuelan Basin

Geochemical characteristics of the organic matter in the thick shaly sequence of the Carapita Formation (lower Miocene) were analyzed in five exploratory wells drilled in the Maturin Subbasin, Eastern Venezuela. The study also includes crude oils from well One, analyzed to establish their origins and possible sources. The quantity and quality of organic matter in the Carapita Formation (% TOC, H/C, HI, OI, and kerogen type) indicate vertical variation. Shales of the Carapita Formation increase in source rock potential from top to bottom. This suggests two facies with different hydrocarbon potential. Maturity parameters ( T max = 428–446°C, %R o = 0.35–0.73%) indicate that the upper section is immature and the lower section is mature. Thus, the lower section is sufficiently mature to generate oil. MATOIL modeling in well One shows that the lower section of the Carapita Formation began to generate oil in Late Miocene time (6.4 m.a.), at a depth of about 15,900 ft (4846 m). Geochemical characteristics of oils from well One indicate generation from a source with abundant higher land plant input. These are the first terrestrial crude oils of this type found in the fold thrust belts of the Maturin Subbassin. GC-MS analyses of saturated hydrocarbon fractions suggest a possible correlation with the terrestrial organic matter in the Carapita Formation.

Sequence Stratigraphy Analysis of Carapita E Lower Miocene; North of Monagas, Venezuela

The Carapita E sandstone is of lower Miocene age, and occurs in the maturin sub-basin within the active thrust belt of the eastern Venezuela tertiary foreland basin. This basin is a prolific hydrocarbon basin and the Carapita sandstones form an important reservoir system. This study is based on an integration of well logs and cores from 56 wells covering an area of more than 39.000 km[sup 2] the sediments were interpreted in terms of depositional environments and sequence stratigraphic patterns. The sandstone unit overlies a type I sequence boundary (25,5 m.y.) and in the central and western part of the study area it is interpreted as a lowstand systems tract, deposited within an incised valley on the shelf. Further east, the sediments were deposited in a coastal to shelf setting and the unconformable sequence boundary is interpreted to pass into a correlative conformity. These more distal shoreline sands from a shelf-perched lowstand prograding wedge which is coeval with the incised valley fill further landward they comprise coastal and nearshore deposits in the form of prograding coastal/deltaic bars and shoreline sands. This detailed sedimentological and sequence stratigraphic study has resulted in improved productivity and optimization of hydrocarbon recovery from the Carapitamore » E sands.« less

Tectonic Provinces and Different Types of Hydrocarbon Traps in the Maturin Subbasin, Eastern Venezuela Basin

During the Oligo-Miocene, the transcollision of the Caribbean and South American plates, caused the northern flank of the Maturin Sub-basin to deform. This geodynamic process transformed the original passive margin basin into a foreland basin. As a result, the tectonic elements that distinguish this basin can be subdivided into three categories, the allochthonous block, the parautochthonous block and thrust belt, and the authochthonous block. The integrated interpretation of 7000 Km of seismic data and 25 wells has enabled us to elucidate the structural framework of the basin. A [open quotes]forward breaking[close quotes] sequence is proposed starting at the Pirital Thrust in the north and ending at the Furrial-Tejero Thrust in the south. However, the presence of a Post-Middle Miocene unconformity, folding of Early-Middle Miocene sediments and the development of a back thrust and its subsequent delta zone strongly suggests that movement towards the north also occurred. Between the Pirital and Tala faults, a lateral ramp known as the Urica fault, separates two zones that have been deformed by different mechanisms. The Urica fault has been interpreted by several authors as a dextral strike-slip fault. Hydrocarbons occur in three areas. The most important province occurs in the parautochthonous blocks with hydrocarbonsmore » occurring in cylindrical folds associated with the thrusts. To the south, conical folds associated with a strike-slip fault zone form good structural traps. The traditional Oficina fields lie still further south, in an area of normal faulting lightly affected by the Oligo-Miocene plate collision to the north.« less

A Biostratigraphic Sequence Analysis in Cretaceous Sediments from Eastern Venezuela

This paper presents the results of a high resolution biostratigraphic study integrated with petrophysic analyses, of the Late Cretaceous sequence in several wells from the Maturin Sub-Basin, Eastern Venezuela. The main objective of this study is to integrate the different faunal and floral assemblages to the sedimentological evolution of the basin using sequential analysis techniques. This technique was applied using mainly terrestrial and marine palynomorphs which were relatively abundant and diverse as compared to the scarcity of foraminifera and nonnofossils. Based on the percentages of abundance and the diversity of the different groups of microfoss it was possible to establish the maximum flooding surfaces and condensation levels which allowed the definition of the possible candidates for the sequence boundaries. On the other hand, the identified bioevents made possible the definition of the chronostratigraphic datums of the sequence under study. The results obtained will contribute to optimize the exploration and development programs of the oil fields in Eastern Venezuela.

Restudy of the Sedimentary Environment of Los Jabillos Formation of Northeastern Venezuela

From the study of five sections located on the southern flank of the Interior Range, we have determined the sedimentary characteristics of Los Jabillos Formation and propose a model for its deposition. During Oligocene time there were changes in the paleoslope direction of the basin, with a predominance of southwest to northeast. The relative variations of sea level, in this part of the Western Venezuelan basin were controlled by subsidence tectonic, which was of major influence in the sedimentation of Los Jabillos Formation. During this epoch and under these conditions, the shelf was covered by sediments from Guayana shield. There were deposits in a wave dominated delta, where the stream-mouth bars were continually reworked into a series of superimposed coastal bars. The littoral currents transported these sands sediments parallel to the coast, causing a decrease in thickness of the units both toward the east and the west. The sandy beds (beach and marine bar), deposits on both sides of stream mouth are preserved in outcrops in Querecual, Oro, and Areo rivers. They are composed of mature quartz sandstone, with good size selection and have, therefore, good permeability. Examples of the marine bar facies have also been studied in wells inmore » the Maturin subbasin where data were obtained from a conventional core.« less

Heterogeneity of the Organic Matter in the Guayuta Group, Eastern Venezuelan Basin

The purpose of this study is to evaluate the organic matter heterogeneities in the Guayuta Group as a principal hydrocarbon source rock in the Eastern Venezuelan Basin. In order to do this, thirteen wells and five work stations on outcrops of the Interior Mountain Belt were analyzed to study the regional and vertical variations in the geochemical characteristics of the organic matter. It is possible to detect significant differences in quality and quantity of the organic matter which could corroborate the regional development of two organic facies from North to South in the Maturin Subbasin. The northern organic facies show excellent characteristics as source rock. The study of vertical distribution of organic matter was carried out in a well of northern part of the Monagas state, which represents the southern organic facies. It shows an irregular input of continental organic matter, thermally immature. Besides the organic matter content was low (around 1.5%) without depth tendencies. These sediments are clastic and bioclastic in contrast with carbonates and pelagic shales of the Guayuta Group in the Interior Mountain Belt. The outcrop samples studied show a high total organic content (2-6%) despite the high maturity determined on kerogen. The systematic study of thismore » geochemical parameter show pseudocyclic relationships with a general tendency to increase toward the bottom of the section. V, Ni, and S determinations could indicate that anoxic conditions were developing toward the North where the marine organic matter was sedimenting. The results of this study are in agreement with paleogeographic model of sedimentation during middle and late Cretaceous, with sources of sediments from South and a progressive depth of the basin toward the North.« less

Assessing the Impacts of Geopressure on Exploration Using Integrated Geological Log Analysis

The occurrence of geopressure anomalies in the El Furrial, Chaguaramal and Boqueron oil fields in the Maturin subbasin of the Eastern Venezuela Basin, and their heterogeneous evolution through geological time has caused significant drilling problems. Identification of these anomalies and isolation of the adjacent subcompacted formations has cost much time and money. There are two main abnormal pressure zones in the Eastern Venezuela Basin. The lower occurs in a thick homogenous marine shale sequence of the Carapita Formation, deposited during late Oligocene-early Miocene times, which was followed by a late Miocene period of uplift and erosion. The upper occurs in marine shales and sandstones of the La Pica Formation, which was transgressively deposited on the eroded Carapita Formation. Using an integrated analysis of lithology and wireline logs, tectonically-influenced sections have been identified within the lower geopressure which have modified its original conditions. Furthermore, the late Miocene unconformity between the two zones controls the occurrence, style and dimension of the upper geopressure. Detailed differential compaction analysis from low to high hydraulic potential intervals through the lithostratigraphic sequence and their association to the unconformity explain the evolution and the current state of geopressure distribution throughout the oil fields. These conclusions are supportedmore » by direct and indirect quantitative pore and fracture pressure gradient analysis. The study identifies areas within these fields (e.g., Central and Eastern Boqueron) which show alterations of the occurrence and behavior of the geopressure. Such knowledge permits rapid modifications to drilling programs, allowing successful exploration and development.« less

Late Cretaceous and Mid-Tertiary Palynological Assemblages from the El Furrial"Area Wells, Venezuela"

The Maturin subbasin, located within the Eastern Venezuelan basin and where the 'El Furrial' field was discovered, has been the subject of great exploration interest, since large amounts of oil were found in the last decade. This paper is part of a multidisciplinary effort to better understand the geology of the area. From the late part of the Paleogene onward, this subbasin developed as a foreland type basin, flanked to the north by a mountain belt: the Serrania del Interior. The stratigraphic nomenclature previously established in the Serrania del Interior was used in subsurface studies, but sometimes with unsuccessful results. An attempt is made in this paper to clarify in part the nomenclature since the biostratigraphic point of view sees the ages determined in subsurface not completely agreeing with those established for the surface formations. The palynological studies carried out on well samples from the 'El Furrial' field have shown the presence of a Late Cretaceous sequence with terrestrial pollen and spores assemblages, and marine microplankton, through which the sequence was dated as Early to Middle Maastrichtian. Toward the top, the sequence contains paleosols, and the palynomorphs are scarce and nondeterminant. The paleoenvironments vary from marine nearshore to subaereal inmore » the Cretaceous. Immediately after a glauconitic section was deposited the first Tertiary (Oligocene) palynomorphs and foraminifers appear, thus showing an important time gap not completely determined as yet. The paleoenvironments suggested for this Tertiary sequence are marine, probably of the shelf of the neritic zone.« less

Cretaceous and Tertiary Palynology Used in the Geological Understanding of the Maturn Sub-Basin, Venezuela

The use of palynology and micropaleontology, two biostratigraphic tools used at Intevep, S.A., have been of great help in the geological understanding during studies done on cores of the Maturin subbasin, a foreland type basin in Eastern Venezuela. Basically, the palynological work consisted of palynological and palynofacies analysis used to determine ages and to interpret the paleoenvironments of deposition in comparison with the interpretations obtained through sedimentological and micropaleontological studies. Palynomorph assemblages and palynofacies are shown for the quiescent Late Cretaceous, for the Tertiary in sequences that are not exactly dated and probably represent the first compressive movements in the basin, and for the Oligocene. In general, in these sequences it was possible to determine marine environments varying from near-shore to inner neritic, and also subaereal paleosols in the Cretaceous. The work highlights the importance of multidisciplinary studies in order to obtain interpretations more in accordance with reality.

Diagenesis and Reservoir Characterization of the Cretaceous-Tertiary Sequence, Eastern Venezuela: ABSTRACT

The giant El Furrial field Maturin subbasin is the most important oil field discovered in Venezuela in the last three decades. The average oil column has a thickness of 400 m and the reservoirs consist of essentially sandy siliciclastic sediments of nearshore-shallow marine origin. The oil's API gravity ranges from light to extra heavy and occurs in a stratified manner in the reservoirs. A total of 1,080 m of core from the producing sequence was studied through x-ray diffraction scanning electron microscopy, and petrography. This data, along with petrophysical measurements, show a clear differentiation between the Upper Cretaceous and the Oligocene reservoirs. The Upper Cretaceous reservoirs are characterized by relatively fine and uniform grain size, subarkosic composition with common volcanic rock fragments, high degree of chemical and mechanical compaction highly illitic mixed-layer I/S assemblage with less than 10% expandable layers, and ubiquitous baroque dolomite. Additionally, porosity and permeability values are persistently low. Clearly, the Cretaceous sediments are diagenetically mature and may indicate diagenetic transformation at greater depths or under a different thermal regime. In contrast the coarser grained Oligocene reservoirs of quartz arenitic composition show a lesser diagenetic overprint, and greater porosity and permeability. Porosity is dominantly secondary due tomore » cement and grain (mostly quartz) dissolution, as well as tectonically induced grain fracturing. Common kaolinite and minor amounts of I/S with up to 20% of expandable layers attest to a lower diagenetic regime than in the Cretaceous reservoirs.« less

Generation and migration of oil in the Maturin Subbasin, Eastern Venezuelan basin

Regional geochemical evaluation of the possible Cretaceous and Tertiary source rocks and oil-oil and oil-source rock correlation studies, have identified the calcareous shales and limestones of the Upper Cretaceous, Querecual and San Antonio Formations of the Guayuta Group, with amorphous marine type II organic matter, as the source rocks for oil in the Maturin Subbasin. The deposition of the source rocks took place on a passive continental margin whilst the generation and migration of oil occurred in a foreland basin during Miocene-Recent times. The oils found in the northern part of the subbasin in different reservoirs (Cretaceous, Eocene, Oligocene, Miocene and Pliocene formations) are of mature, marine type with or without bacterial alteration. Geochemical data indicate that some of these oils were mixed with the highly altered residue of a previously accumulated oil. Based on maturity and migration modelling, it appears that the oils essentially migrated from active, Upper Miocene-Recent, kitchens in the Cretaceous source rocks below the main thrusts. The highly altered residue was formed during the Upper Miocene by the biodegradation of oil migrating during the Middle and Upper Miocene. Large areas of mature to overmature Cretaceous source rocks (now exposed in the Interior Mountain Front) were in active oil kitchens during the Lower and Middle Miocene before being involved in thrusting around 12 Myr BP. Long distance (150–300 km) southward lateral migration of oils during the Middle Miocene gave rise to the altered moderately mature marine oils in the shallow Miocene stratigraphic reservoirs of Cerro Negro (Orinoco Oil Belt) in the southern fringe of the subbasin.