Maracaibo Basin Research Articles

Deep structure of the Mérida Andes and Sierra de Perijá mountain fronts, Maracaibo Basin, Venezuela

The Maracaibo Basin is a 50,000-km2 (19,305-mi2) topographic depression bounded to the east and south by the Mrida Andes of Venezuela and to the west by the Sierra de Perij of Venezuela and Colombia. Both uplifted mountain blocks expose Paleozoic basement rocks and Mesozoic–Cenozoic carbonate and clastic rocks that were mainly folded and thrusted by regional shortening in the Paleogene and late Neogene. Using geologic maps, seismic reflection data, and wells from the steep mountain front areas, we test different structural models of how late Paleogene–Neogene convergence is accommodated along subsurface faults and folds at the mountain fronts. Seismic imaging of the deep (5-km; 3.1 mi) structure of both Maracaibo mountain fronts shows basinward-dipping monoclines with stratal dips ranging from 20 to overturned and an almost complete lack of faulting in the basin-edge monocline or in adjacent, horizontally bedded strata of the Maracaibo Basin. Seismic data reveal the presence of one or more triangle zones at depth along both the Mrida Andes and Sierra de Perij that exhibit characteristic thrust-wedge geometries. The lower part of the wedge is defined by imbricate thrust faults dipping beneath the mountain block and involving the Paleozoic basement. The upper part of the wedge is defined by a basinward-dipping thrust associated with fault-propagation folds at the surface and an overlain basin-edge monocline. The creation of the steep to overturned dips of the monocline is attributed to the effects of 6–10 km (3.7–6.2 mi) of shortening along the lower zone of imbricated thrust faults. This 6–10 km (3.7–6.2 mi) of shortening, calculated from triangle zones of both the Sierra de Prija and Mrida Andes, is significantly less than regional estimates from 12 to 60 km (7.4 to 37 mi) of shortening inferred by previous workers from regional balanced cross sections that assume low-angle thrust-type geometries. We propose that a pop-up style of deformation related to the inversion of Jurassic rift features may be a more realistic interpretation of the convergent fault systems that have controlled uplift of the Mrida Andes and Sierra de Perij. Inversion of relatively steep, basement normal faults at the edges of and within both ranges may explain lesser amounts of observed shortening.

Deeply buried, Early Cretaceous paleokarst terrane, southern Maracaibo Basin, Venezuela

Cretaceous carbonate rocks formed an extensive passive-margin section along the northern margin of the South American plate and are now found in outcrops in elevated and deformed ranges like the Mrida Andes and Sierra de Perij. Regional seismic profiles correlated with well data show that a 300-m (984-ft)-thick Cretaceous carbonate platform underlies all of the Maracaibo Basin of western Venezuela. We examined the Cretaceous carbonate section beneath the southern Maracaibo Basin using a 1600-km2 (617-mi2) area of three-dimensional (3-D) seismic reflection data provided by Petrleos de Venezuela, S. A., along with wells to constrain the age and environments of seismic reflectors. Well data allow the identification and correlation of the major lithologic subsurface formations with formations described from outcrop studies around the basin edges. Seismic reflection time slices at a depth range of 3.7–4.5 s (5–7 km; 3.1–4.3 mi) reveal the presence of a prominent, irregular reflection surface across the entire 3-D study area that is characterized by subcircular depressions up to about 600 m (1968 ft) wide and about 100 m (328 ft) deep. We interpret the subcircular features as sinkholes formed when the Lower Cretaceous carbonate platform was subaerially exposed to weathering in a tropical climate. The scale of the observed circular features is consistent with dimensions of limestone sinkholes described from modern karst settings. Correlation of the inferred karst horizon with well logs shows that the paleokarst horizon occurs within the shallow-water carbonate rocks of the Aptian Apn Formation. We infer that the karst formed during an Aptian eustatic sea level fall described from Aptian intervals in other parts of the world, including the Gulf of Mexico. The Aptian paleokarst zone provides a previously unrecognized zone of porosity for hydrocarbons to accumulate beneath the Maracaibo and perhaps other basins formed above the extensive passive margin of northern South America.

Cretaceous to Holocene structural and stratigraphic development in south Lake Maracaibo, Venezuela, inferred from well and three-dimensional seismic data

The subsurface of the southern part of the Maracaibo Basin has been studied in much less detail than the central and northern basin because hydrocarbon-rich rocks of Cretaceous and Paleogene age are deeply buried to a depth of 4.5 km (2.7 mi) by a thick Neogene clastic section eroded off of surrounding mountain ranges. In this article, we describe the results of the interpretation of 1600-km2 (617-mi2) three-dimensional (3-D) seismic reflection data centered on this horizontal to monoclinally southwestward-dipping middle Cretaceous to Holocene section adjacent to a zone of northwestward overthrusting along the mountain front of the Mrida Andes. To establish the structural and stratigraphic history of the 3-D seismic study area, a series of time slices were constructed at key horizons through the 3-D seismic volume. The ages of units in these time slices were correlated to stratigraphic formations known from outcrops at the basin edges, well data from the 3-D seismic study area, and subsurface correlation with better studied areas in the northern basin. The type and orientations of structures along each time horizon were mapped to constrain the successive structural and stratigraphic events that affected the different tectonosequences in the southern Maracaibo Basin. The main tectonic events recorded both by structures and unconformity-bound tectonosequences include the following events from youngest to oldest: (1) Pliocene–Holocene shortening produced by the uplift of the Mrida Andes and overthrusting at the mountain front has produced an approximately 6-km (3.7-mi)-deep localized foredeep basin; the axial traces of these folds trend northeast-southwest and parallel the strike of a set of normal faults; all structural features are consistent with regional northwest-southeast shortening. A low-angle thrust fault responsible for the formation of the main northeast-southwest fold is inferred to continue downdip as a flat-lying thrust that may be continuous with low-angle thrust faults observed along the northern Mrida Andes mountain front. The level of thrust detachment occurs at the Cretaceous Coln shale and accommodates a negligible amount of shortening (500 m; 1640 ft). (2) Late Miocene–Pliocene shortening caused by uplift of the Sierra de Perij and the Mrida Andes produced an inversion of north-south–striking normal faults and folds in a parallel orientation. The axial traces of these folds and the trend of inverted structures coincide with the trends of the most prolific Eocene sandstone reservoirs in the central and northern Lake Maracaibo areas. (3) Late Paleocene–middle Eocene formation of a foreland basin is related to the southward overthrusting at the northeastern margin of the basin; normal faults with an average trend of S37E record flexure of the basin during this event; faults also include the southern splayed termination of the left-lateral Icotea strike-slip fault. (4) The termination of the Icotea fault in the south lake area is consistent with the small amount of left-lateral strike-slip displacement previously inferred from the geometry of the Eocene Icotea pull-apart basin in central Lake Maracaibo.

An overview of the petroleum system of Maracaibo Basin

The geologically complex Maracaibo Basin in northwestern Venezuela is one of the most prolific hydrocarbon basins in the world. Having a basinal area of 50,000 km2 (19,300 mi2), the basin has produced more than 30 billion bbl of oil, with estimated recoverable oil reserves of more than 44 billion bbl. The central elements of the petroleum system of the basin include (1) a world-class source rock (Upper Cretaceous La Luna Formation), deposited on a shelf-to-slope environment under anoxic conditions and modified by intermittent oxygenated periods and tectonic events; (2) high-quality clastic reservoir rocks deposited in Eocene and Miocene fluviodeltaic settings; (3) two main periods of rapid tectonic subsidence responsible for two pulses of voluminous hydrocarbon generation, first, during Paleogene Caribbean–South American oblique plate collision and, second, during the Neogene uplift of the Sierra de Perij–Mrida Andes; and (4) lateral and vertical migration of oil along strike-slip, normal, and inverted faults, as well as a regional unconformity of late Eocene–Oligocene age. The maturation, migration, and trapping of hydrocarbons were closely controlled by the tectonic evolution of the Maracaibo Basin. During the Paleogene, the development of a foredeep along the northeastern margin of the basin and the strike-slip reactivation of the rift-related Jurassic faults on the Maracaibo platform controlled the early structural setting of the source and reservoir rocks. Hydrocarbons migrated updip from source rocks beneath the north-northeastern margin of the basin along north-south strike-slip faults and into overlying Eocene clastic reservoirs in the south-central parts of the basin. The second period of the Maracaibo Basin petroleum system developed during subaerial exposure of most of the Maracaibo Basin during Oligocene–Miocene uplift of the adjacent Sierra de Perij and Mrida Andes. Uplift of mountain ranges surrounding the basin folded and depressed the interior of the basin to form the extensive Maracaibo syncline. Because of the formation of the Maracaibo syncline, oil generation began in the central and southern parts of the synclinal basin and migrated northward. Hydrocarbons migrated up the flanks of the Maracaibo syncline along reactivated strike-slip faults and into Miocene rocks adjacent to the uplifted mountain ranges. Escaping oil has formed numerous surface seeps along the edges of the Maracaibo Basin.

Sequence-stratigraphic analysis of Eocene clastic foreland basin deposits in central Lake Maracaibo using high-resolution well correlation and 3-D seismic data

Eocene clastic rocks of the Maracaibo Basin were deposited in an asymmetrical foreland basin formed during the oblique Paleogene collision between the Caribbean and South American plates. In this study, we use more than 300 wells and 2000 km2 (772 mi2) of seismic data in the central Maracaibo Basin to produce a detailed sequence-stratigraphic interpretation of the Eocene Maracaibo foreland basin. The base of the Eocene stratigraphic succession in the central Maracaibo area is characterized by an approximately 250-m (820-ft)-thick, aggradational succession of fluviodeltaic sandstone overlain by an approximately 600-m (1968-ft)-thick retrogradational succession of shallow-marine shale and sandstone containing minor progradational units. The upper part of the foreland basin sequence is marked by an approximately 100-m (328-ft)-thick aggradational succession of fluviodeltaic sandstone. In the approximately 1000-m (3280-ft)-thick Eocene section, we interpreted 17 parasequence sets, 6 genetic sequences, and 1 depositional sequence. Only one classic sequence boundary was interpreted within the Eocene section that marks the boundary between the retrogradational shallow-marine section and the overlying aggradational fluviodeltaic succession. Based on the stratigraphic architecture and thickening trends of several of the parasequence sets, we conclude that the main source of clastic sedimentation was located on the South America craton south of the Maracaibo Basin, instead of along the thrusted, north-northeastern margin of the basin as proposed by previous workers. A lack of recognition of classic sequence boundaries suggests that Eocene clastic rocks of the central Maracaibo foreland basin were not subaerially exposed during most of the Eocene, and that their stratigraphic architecture was controlled by tectonic subsidence related to thrusting along the northeastern edge of the foreland basin. Eustasy was not an important control on the stratigraphic evolution of the foreland basin until its middle Eocene aggradational period that marked the end of foreland basin subsidence. Well logs and three-dimensional seismic data show that depositional environments on the Eocene delta plain and shelf of the central Maracaibo foreland basin were dominated by fluvial and tidal processes that are similar to modern depositional processes of the Orinoco delta in eastern Venezuela.

Petrophysical and seismic properties of lower Eocene clastic rocks in the central Maracaibo Basin

High-resolution, well-log cross sections are used to map the continuity of hydrocarbon-bearing sandstone reservoirs in the Centro Lago fields, central Maracaibo Basin, Venezuela. Because most sandstone reservoirs are below the limit of seismic resolution, reservoir mapping relies on detailed well-log correlations. Seismic data are used mainly to define the structural framework. A feasibility analysis based on the correlation of seismic data and petrophysical properties of Eocene clastic rocks was conducted to demonstrate that lithology and fluid content can be differentiated by acoustic impedance known from well-log data. However, low vertical resolution of the three-dimensional seismic data made available for this study (25 Hz dominant frequency) is not adequate to constrain the lateral and vertical continuity of the Eocene reservoirs. The main sandstone reservoirs in lower Eocene clastic rocks of the Centro Lago fields are found in distributary channels and tidal sand-bar facies. These reservoirs provide good-quality fluid-flow units. Channel complexes and sand bars are concentrated along the crest of a north-northeast–trending anticline bounded by parallel left-lateral strike-slip faults. Reservoirs are generally compartmentalized by transverse northwest-southeast–striking faults that intersect the main anticlinal axis. Detailed high-resolution well-log cross sections and mapping, integrated with the structural framework interpreted from the seismic data, indicate the presence of untested oil plays in the Centro Lago fields. These new plays are located along the flanks of the main anticline and are found in Eocene tidal bars in the central and northern areas of the Centro Lago fields.

Interpreting fracture development from diagenetic mineralogy and thermoelastic contraction modeling

Four sets of thin-section scale, Mode I (open mode), cemented microfractures are present in sandstone from the Eocene Misoa Formation, Maracaibo basin, Venezuela. The first set of microfractures is intragranular (F1), formed early during compaction and are filled with quartz cement precipitated at temperatures equal to or higher than 100 °C. The second set of microfractures (F2) is cemented by bituminite–pyrite, formed at temperatures between 60 and 100 °C, and are associated with kerogen maturation and hydrocarbon migration from underlying overpressured source rocks. The third set of microfractures (F3) is fully cemented by either quartz cement or calcite cement. The former has fluid inclusion homogenization temperatures between 149 and 175 °C. These temperatures are mostly higher than maximum burial temperatures (∼160 °C), suggesting that upward flow, caused by a pressure gradient, transported silica vertically which crystallized into the fractures. Upward decompression may have also caused a P CO 2 drop, which, at constant temperature, allowed simultaneous carbonate precipitation into the third microfracture set. The fourth set of thin-section scale microfractures (F4) is open or partially cemented by siderite–hematite and other iron oxides. The presence of hematite and iron oxides in microfractures is evidence for oxidizing conditions that may be associated with the uplift of the Misoa formation. In order to time and place constraints on the depth of formation of the fourth set of microfractures, we have coupled published quartz cementation kinetic algorithms with uniaxial strain equations and determined if, in fact, they could be associated with the uplift of the formation. Our results suggest that thermoelastic contraction, caused by the formation's uplift, erosion, and consequent cooling is a feasible mechanism for the origin of the last fracture set. Hence, we infer that meteoric water invasion into the fractures, at the end of the uplift, cause the precipitation of oxides and the transformation of siderite to hematite.

Geochemistry of the Mirador Formation (Late Eocene-Early Oligocene), southwestern Venezuela: Chemostratigraphic constraints on provenance and the influence of the sea level

A geochemical study was carried out on Eocene-Lower Oligocene sedimentary rocks (the Mirador Formation) from the southern Lake Maracaibo Basin, western Venezuela. The goal of this work was the identification of source rock composition and tectonic settings of source areas, and their temporal changes. The geochemistry was also used to investigate attributes of the paleoenvironment. Eighty-nine rock samples were collected and the bulk inorganic geochemistry (Al2O3, SiO2, TiO2, Fe2O3, MnO, CaO, MgO, K2O, Li, B, Sc, V, Cr, Ni, Zn, Rb, Y, Mo, La, Ce and Th) was determined by ICP-AES; total sulfur was determined in a LECO SC-432 analyzer. Multivariate statistical tools were employed to evaluate the correlation between the different variables. Through cluster-constrained analysis, four subdivisions, or main chemofacies, were defined and correlated with the significant lithologic units. The tectonic setting for the source rock could be a passive margin, and a mixed source was inferred, consisting of recycled sedimentary rocks and igneous and/or meta-igneous rocks of predominantly felsic composition. The most marine-influenced lithological units occur at the bottom of the section.

An Integrated Calcareous Microfossil Biostratigraphic and Carbon-Isotope Stratigraphic Framework for the La Luna Formation, Western Venezuela

Other| October 01, 2003 An Integrated Calcareous Microfossil Biostratigraphic and Carbon-Isotope Stratigraphic Framework for the La Luna Formation, Western Venezuela LINDA M. DE ROMERO; LINDA M. DE ROMERO 1Department of Geological Sciences, CB #3315, University of North Carolina, Chapel Hill, NC 27599, deromero@email.unc.edu Search for other works by this author on: GSW Google Scholar IRENE M. TRUSKOWSKI; IRENE M. TRUSKOWSKI 2Petróleos de Venezuela, PDVSA, Exploración, Apdo. 829, Caracas 1010A, Venezuela Search for other works by this author on: GSW Google Scholar TIMOTHY J. BRALOWER; TIMOTHY J. BRALOWER 3Department of Geological Sciences, CB #3315, University of North Carolina, Chapel Hill, NC 27599 * Current address: Department of Geosciences, Pennsylvania State University, University Park, PA 16802. Search for other works by this author on: GSW Google Scholar JAMES A. BERGEN; JAMES A. BERGEN 4Department of Geological Sciences, CB #3315, University of North Carolina, Chapel Hill, NC ** Current address: BP America, Inc., 501 Westlake Park Blvd., Houston, TX 77079. Search for other works by this author on: GSW Google Scholar OSCAR ODREMAN; OSCAR ODREMAN 5Escuela de Ingeniería Geológica, Universidad de Los Andes, Mérida, Venezuela Search for other works by this author on: GSW Google Scholar JAMES C. ZACHOS; JAMES C. ZACHOS 6Earth Sciences Department, University of California, Santa Cruz, CA 95064 Search for other works by this author on: GSW Google Scholar FRANCIA A. GALEA-ALVAREZ FRANCIA A. GALEA-ALVAREZ 7Petróleos de Venezuela, PDVSA, Exploración, Apdo. 829, Caracas 1010A, Venezuela Search for other works by this author on: GSW Google Scholar Author and Article Information LINDA M. DE ROMERO 1Department of Geological Sciences, CB #3315, University of North Carolina, Chapel Hill, NC 27599, deromero@email.unc.edu IRENE M. TRUSKOWSKI 2Petróleos de Venezuela, PDVSA, Exploración, Apdo. 829, Caracas 1010A, Venezuela TIMOTHY J. BRALOWER * Current address: Department of Geosciences, Pennsylvania State University, University Park, PA 16802. 3Department of Geological Sciences, CB #3315, University of North Carolina, Chapel Hill, NC 27599 JAMES A. BERGEN ** Current address: BP America, Inc., 501 Westlake Park Blvd., Houston, TX 77079. 4Department of Geological Sciences, CB #3315, University of North Carolina, Chapel Hill, NC OSCAR ODREMAN 5Escuela de Ingeniería Geológica, Universidad de Los Andes, Mérida, Venezuela JAMES C. ZACHOS 6Earth Sciences Department, University of California, Santa Cruz, CA 95064 FRANCIA A. GALEA-ALVAREZ 7Petróleos de Venezuela, PDVSA, Exploración, Apdo. 829, Caracas 1010A, Venezuela Publisher: SEPM Society for Sedimentary Geology Accepted: 24 Jun 2003 First Online: 03 Mar 2017 Online ISSN: 1938-5323 Print ISSN: 0883-1351 Society for Sedimentary Geology PALAIOS (2003) 18 (4-5): 349–366. https://doi.org/10.1669/0883-1351(2003)018<0349:AICMBA>2.0.CO;2 Article history Accepted: 24 Jun 2003 First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation LINDA M. DE ROMERO, IRENE M. TRUSKOWSKI, TIMOTHY J. BRALOWER, JAMES A. BERGEN, OSCAR ODREMAN, JAMES C. ZACHOS, FRANCIA A. GALEA-ALVAREZ; An Integrated Calcareous Microfossil Biostratigraphic and Carbon-Isotope Stratigraphic Framework for the La Luna Formation, Western Venezuela. PALAIOS 2003;; 18 (4-5): 349–366. doi: https://doi.org/10.1669/0883-1351(2003)018<0349:AICMBA>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyPALAIOS Search Advanced Search Abstract The biostratigraphy of the La Luna Formation has long been in dispute, despite the importance of this unit as the most important source rock for hydrocarbons in Venezuela. In this paper, calcareous microfossil biostratigraphy combined with carbon-isotope stratigraphy provides a stratigraphic framework for the formation, permitting revision of temporal and spatial patterns of deposition of organic-rich sediments. Detailed studies were conducted on a cored borehole and five outcrop sections distributed across the Maracaibo Basin of western Venezuela.Planktic foraminifera have fair to good preservation, and nannofossils are poorly preserved. Many of the Cenomanian to Campanian planktic foraminiferal marker species are present, permitting the application of a traditional zonal scheme. An informal nannofossil biostratigraphic zonal scheme, based primarily on dissolution-resistant species, has been developed. Integration of these zonal schemes has enabled the correlation of changes in carbon-isotope ratios to the global C-isotope stratigraphy. The results have been used to estimate temporal variation in sedimentation rates as well as to reconstruct depositional patterns across the Maracaibo Basin. Deposition began in the eastern and northwestern part of the Maracaibo Basin in the middle Cenomanian and progressed towards the south and west, reaching the southwestern corner by the middle Coniacian. Although the uppermost part of the formation could not be dated, deposition in the eastern basin continued at least into the Coniacian and in the western basin at least until the middle–late Santonian. Sedimentation rates were highly variable with a period of condensed sedimentation at the Cenomanian–Turonian boundary. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Late Cretaceous Anoxia and Lateral Microfacies Changes in the Tres Esquinas Member, La Luna Formation, Western Venezuela

Other| October 01, 2003 Late Cretaceous Anoxia and Lateral Microfacies Changes in the Tres Esquinas Member, La Luna Formation, Western Venezuela MARIEN PARRA; MARIEN PARRA 1Petróleos de Venezuela, PDVSA, División Oriente, Apdo. 3426, Puerto La Cruz 6023, Venezuela parramo@pdvsa.com Search for other works by this author on: GSW Google Scholar LORENA MOSCARDELLI; LORENA MOSCARDELLI 2University of Texas at Austin, University Station C1000, Austin, TX, 78712 Search for other works by this author on: GSW Google Scholar MARIA ANTONIETA LORENTE MARIA ANTONIETA LORENTE 3GeoAmbio, Apdo 281, Jumilla 30520, Spain Search for other works by this author on: GSW Google Scholar PALAIOS (2003) 18 (4-5): 321–333. https://doi.org/10.1669/0883-1351(2003)018<0321:LCAALM>2.0.CO;2 Article history accepted: 01 Jun 2003 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation MARIEN PARRA, LORENA MOSCARDELLI, MARIA ANTONIETA LORENTE; Late Cretaceous Anoxia and Lateral Microfacies Changes in the Tres Esquinas Member, La Luna Formation, Western Venezuela. PALAIOS 2003;; 18 (4-5): 321–333. doi: https://doi.org/10.1669/0883-1351(2003)018<0321:LCAALM>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyPALAIOS Search Advanced Search Abstract The Tres Esquinas Member of the La Luna Formation is a glauconite-rich phosphorite unit associated with changes that took place in depositional environments in the Maracaibo Basin at the end of the Late Cretaceous. The unit marks the end of La Luna Formation sedimentary cycle. This paper presents results of petrographic studies of samples from two sections, one outcrop (Río Guaruries), and one well core (Perijá), and from seismic profiles across the Perijá and Colón areas in the Maracaibo Basin.The Tres Esquinas Member is three meters thick in both locations. In the Río Guaruries outcrop, it contains an abundant foraminiferal fauna in addition to common allochemical material. Perijá core samples contain voluminous carbonate matrix, scarce allochemical materials, rare foraminifera, and poor definition of mineral facies, which may reflect higher-energy conditions on the sea floor.The deposition of the Tres Esquinas Member resulted from altered sea-floor topography during an episode of intense tectonic activity in the eastern part of the Maracaibo Basin. The modified shelf configuration ended anoxic conditions on the sea floor and led to increased erosion characteristic of the Tres Esquinas Member. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geochemical implications of trace elements and sulfur in the saturate, aromatic and resin fractions of crude oil from the Mara and Mara Oeste fields, Venezuela

Ten trace elements (Cr, Zn, Fe, Mn, Cu, Co, Ni, Mo, V and Sr) and sulfur were determined in the saturate, aromatic and resin fractions of 15 crude oils from Mara (DM) and Mara Oeste (DMO) fields of the Maracaibo Basin, Venezuela. The oils studied are classified as unaltered or altered by biodegradation. In the altered oil, the depletion of n-alkanes, the absence of isoprenoids and the presence of steranes and hopanes unaltered by biodegradation are indicative of moderate biodegradation. The elements Zn, Fe, Mn, Cu, Ni, and Sr were detected in the saturated hydrocarbon fraction; Cr and V were detected in the aromatic fraction in addition to the above elements; whereas the elements detected for the resin fraction were Cr, Zn, Fe, Cu, Ni, Mo, V, and Sr. Co was not detected in any fractions of the oils analyzed. Sulfur was found in all fractions of the oils studied. It was proposed that Fe, Zn, Sr, and Mn could have entered the oil during migration or Fe, Zn and Mn as pollutants during oil extraction. Cr and Cu may be of biological origin and Mo could be incorporated into the reservoir through bacteria. Only S, V, and Ni in the resin fractions can be used as indicators of the origin and correlation of Mara and Mara Oeste oils. Based on the results obtained in this work, it can be established that the V/(V+Ni) ratio in the resin fraction can be used as a correlation parameter, for these oils.

Three-dimensional structural architecture and evolution of the Eocene pull-apart basin, central Maracaibo basin, Venezuela

Three-dimensional seismic mapping of interpretative sub-surface time slice sections, incorporating age data and detailed structural observations places constraints on the structural architecture and stratigraphic evolution of the Icotea pull-apart basin in the central Maracaibo basin, Venezuela. All four fault-bounded sides of the Icotea basin, its ∼3-km-thick Eocene basin fill of clastic sediments and faults pre-dating the opening of the basin can be mapped in three dimensions. The development of the Icotea basin provides an excellent observational basis for understanding the structural history of three sets of regionally extensive faults and testing two models for the formation of pull-apart basins in general. Three main fault trends produced at different periods in the formation of the Cretaceous–Recent Maracaibo basin are mapped using 3D seismic data in the central part of the basin: (1) N–NE-striking normal faults, including the Icotea fault; these faults formed originally during Late Jurassic–Early Cretaceous rifting and reactivated as left-lateral strike–slip faults during late Paleocene–Eocene convergent deformation between the Caribbean and South American plates; (2) NW striking faults formed by late Paleocene–Eocene overthrusting of the Caribbean plate and downward flexure of the South American plate to create a major foreland basin depocenter; and (3) NE-striking normal faults, of pre-Cretaceous age, were reactivated during late Paleocene–Eocene plate convergence. Three-dimensional architecture of the Icotea basin interpreted from five time slices through the basin and its flank areas supports the simple pull-apart model for the Eocene opening of the Icotea basin. The amount of extension across the suite of normal faults ranges between 0.8 and 2.25 km. This range of offset is consistent with some previous estimates of minor left-lateral displacement along the Icotea fault but is inconsistent with either low-angle or high-angle thrusting during Eocene time, as inferred from previous interpretations of widely spaced two-dimensional seismic lines. The normal faults that formed the pull-apart basin reactivated pre-existing faults due to plate flexure.

Seismic facies analysis based on 3D multiattribute volume classification, La Palma Field, Maracaibo, Venezuela

La Palma Field, in the southwest corner of Maracaibo Basin (Zulia State, Venezuela) approximately 300 km south of the city of Maracaibo, is in a license block called Colon Unit (Figure 1). The field is operated by Tecpetrol (Argentina), which leads a consortium of three oil companies that also includes CMS Oil & Gas (U.S.) and Coparex (France).

Organic geochemistry of heavy/extra heavy oils from sidewall cores, Lower Lagunillas Member, Tia Juana Field, Maracaibo Basin, Venezuela

The study of 22 oils from sidewall cores taken at different depths in the Lower Lagunillas Member, well LSJ-AB, Tia Juana Field, Maracaibo Lake is presented, with the purpose of predicting the intervals that present the best crude oil quality. Differences were detected in the biodegradation levels of the studied samples, which are correlated with the depth at which the sidewall core was taken. The API gravity was considered for the oils from each sidewall core and it was found that toward the top of the sequence, the oils have an API gravity of 10.6–11.2°, while toward the base part of the sequence, the well produces extra heavy oils with an API gravity that varies between 8.2 and 8.7°.

Permeability prediction in shaly formations: The fuzzy modeling approach

This paper shows how fuzzy rule‐based systems help predict permeability in sedimentary rocks using well‐log responses. The fuzzy rule‐based approach represents a global nonlinear relationship between permeability and a set of input log responses as a smooth concatenation of a finite family of flexible local submodels. The fuzzy inference rules expressing the local input‐output relationships are obtained automatically from a set of observed measurements using a fuzzy clustering algorithm. This approach simplifies the process of constructing fuzzy systems without much computation effort. The benefits of the methodology are demonstrated with a case study in the Lake Maracaibo basin, Venezuela. Special core analyses from three early development wells provide the data for the learning task. Core permeability and well‐log data from a fourth well provide the basis for model validation. Numerical simulation results show that the fuzzy system is an improvement over conventional empirical methods in terms of predictive capability.

A high-frequency sequence study: a Miocene deltaic and estuarine succession in the Eastern Maracaibo Composite Foreland Basin, Western Venezuela

ABSTRACT The Maracaibo Basin in northwestern Venezuela, one of the most prolific oil-producing areas in the world, is a composite foreland basin limited by foredeeps both to the south and west, as well as by piggy-back basins and half-grabens in the east. The western foredeep is part of a retro-foreland basin associated with the Oligocene-Miocene uplifted Sierra de Perija. The La Rosa and the lower Lagunillas formations (lower-middle Miocene) were deposited in the distal portion of this retro-foreland basin. Cores and well logs allow the identification of tidally influenced fluvial, tidal flat, central basin and embayment, shoreface and offshore, lagoonal and organic rich deposits, as well as zones of subaerial exposure within regressive deltaic or strandplain, and transgressive barrier bar or estuarine environments. Core-log calibrations and the identification of stacking patterns and key stratigraphic surfaces allow two types of high-frequency stratigraphic sequences to be recognized. Most sequences are of a simple transgressive-regressive type, implying continuous but irregular rates of relative rise of sea level. However, some have a lowstand segment, implying a relative sea level fall associated with the regressive-to-transgressive turnaround. The repetition of high-frequency sequences, each one of duration less than 0.5 m.y., is believed to have been controlled by high subsidence rates in the basin. The basinward and landward stepping of sequences is interpreted in terms of milder and strong advance of the western thrust-belt wedge and the accompanying variable migration of a forebulge to the east. End_Page 3--------------------------

ABSTRACT: New Geologic Model for the Cretaceous Reservoir of the La Concepci&amp;oacute;n Mature Oil Field, Maracaibo Basin, Venezuela

ABSTRACT: New Geologic Model for the Cretaceous Reservoir of the La Concepci&amp;oacute;n Mature Oil Field, Maracaibo Basin, Venezuela

ABSTRACT: 3-D Seismic Reflection Interpretation of the Southern End of the Icotea and VLE Fault Trends in the Southern Maracaibo Basin, Venezuela

The Icotea and VLE faults are two of the major faults in the Maracaibo Basin, which are known from seismic and well data to have been most active during Paleogene time. Both faults have linear traces over distances up to 100 km, are deeply buried by largely unfaulted Neogene sedimentary rocks, and are associated with localized continental growth strata of Paleogene age along their traces. The origin and displacement history of both faults has remained controversial partly because of along-strike complexities in fault structure, use of highly exaggerated 2D seismic lines, and the lack of synoptic views of the entire fault systems. Interpretations range from east-dipping basement-involved thrust faults to sub-vertical left-lateral strike-slip faults controlling pull-apart basins. In this study we use regional 2D seismic data crossing both faults and 3D seismic data covering a 1600 km2 area of the southern basin to describe the structures associated with the terminations of both faults. These data show that both faults are inverted normal faults first formed during late Jurassic-early Cretaceous. The abrupt termination of both faults in the southern part of the basin probably corresponds to the southern ends of two parallel rift structures. While we cannot rule out strike-slip motion along both faults, their abrupt terminations and lack of termination structures suggest that strike-slip displacement is probably small. In addition, we cannot rule out low angle thrust tectonics in the genesis of both faults since thrusts are confined to the basin edges and are not observed in the basin center.

High-impact palynology in petroleum geology: Applications from Venezuela (northern South America)

This article documents the application of high-impact palynology (HIP) in the Maracaibo Basin of Venezuela and its influence on such exploration and production aspects as regional planning and strategies, risk reduction, optimal drilling decisions and investment, petroleum-system modeling, new discoveries, and secondary recovery by fluid injection, among others. High-impact palynology has been defined as the coupling of high-resolution sequence biostratigraphy, multidisciplinary work, and the alignment of palynology with the attainment of business goals. The first part of this article explains the high-resolution ecostratigraphic methods used and the concept of integrated work applied. The second part of the article shows the results obtained in selected case studies, which illustrate the advantages of HIP. Among the most relevant studies are high-resolution ecostratigraphic frames at a basin level, timing between structural trap formation and oil migration, differentiation of petroleum systems in adjacent reservoirs, the concept of palynoblocks in structurally complex areas to estimate missing sections, stratigraphical models for exploratory wells with better predictions of target horizons, fine-scale reservoir correlations, and discovery of new reservoirs. The use of HIP in other areas is recommended, with palynology as a common in-house practice within multidisciplinary teams formed especially for each specific task.

ABSTRACT: Deeply Buried, Mid-Cretaceous Limestone Karst Surface, Southern Maracaibo Basin, Inferred from 3D Seismic Reflection Data

ABSTRACT: Deeply Buried, Mid-Cretaceous Limestone Karst Surface, Southern Maracaibo Basin, Inferred from 3D Seismic Reflection Data