Middle Eocene Carbonate Research Articles

Diagenetic Evolution of the Middle Eocene Carbonates (Mokattam Formation), Eastern Cairo-Suez District, Egypt

Diagenetic Evolution of the Middle Eocene Carbonates (Mokattam Formation), Eastern Cairo-Suez District, Egypt

IMPACT OF DIAGENESIS ON RESERVOIR QUALITY IN RAMP CARBONATES: GIALO FORMATION (MIDDLE EOCENE), SIRT BASIN, LIBYA

The reservoir quality of Middle Eocene carbonates in the intracratonic Sirt Basin is strongly influenced by depositional facies and various diagenetic processes. Based on data from core samples and well logs from five boreholes in the Assumood and Sahl gasfields in the subsurface of the north‐central Sirt Basin, six major carbonate facies (and fourteen microfacies) are distinguished in the Middle Eocene Gialo Formation (thickness ∼1100 ft/335 m). Wackestones‐packstones dominate the Gialo Formation with abundant grains of larger benthic foraminifera, especially nummulites, in a matrix composed mostly of finely comminuted bioclastic material. Sediments were deposited as a mosaic of facies on a broad carbonate ramp in moderate‐energy nummulite‐packstone banks, with locally restricted back‐bank lagoons and fore‐bank areas of foraminiferal debris passing to offshore lime mudstones.Marine diagenesis was minor with micritization of bioclasts and rare vadose marine fibrous cements. Sparry calcite cements are ubiquitous and were precipitated during shallow to moderate burial from seawater and/or meteoric water, the latter largely unaffected by surface‐pedogenic processes. This is indicated by the fabrics and pre‐ and post‐compaction precipitation of drusy spar and echinoderm overgrowths, and supported by their bright‐to‐dull luminescence and low negative δ18O signatures (‐2.68 to ‐4.16%° PDB). Some early neomorphic alteration of calcitic bioclasts is suggested by bright CL and marine to low negative δ18O values (‐1.06 to –3.93%° PDB). Bioclastic grains have δ13C values ranging between 0.76 and 1.19%° PDB, interpreted as marine signatures. Similar low positive δ13C values of the cements (0.9 to 1.05 %°) indicate a source of carbonate from dissolution of grains and/or seawater/meteoric water, but without any near‐surface/soil effects. Dissolution of grains and matrix, notably originally aragonitic grains but also the originally high‐Mg calcite nummulites, was a major porosity‐enhancing process, and took place from shallow to moderate burial depths. Fractures may locally have increased porosity too.Shallow‐water packstones/rudstones containing both primary intergranular and secondary biomouldic porosity have the best reservoir quality and these are concentrated in the upper parts (top ∼100 ft/30 m) of the wells, with porosity‐permeability decreasing downwards. This trend relates to the broad, large‐scale facies pattern of more mud‐dominated facies giving way upwards to more grainy, nummulite‐dominated facies as a result of an overall shallowing of the depositional environment. In addition, the influx of meteoric waters in the upper part of the Gialo platform, before drowning and deposition of the overlying Augila Shale, increased porosity through dissolution. This integrated study has helped in understanding the reservoir heterogeneity and hydrocarbon potential of the Gialo carbonates.

Meteoric diagenesis of Upper Cretaceous and Paleocene-Eocene shallow-water carbonates in the Kruja Platform (Albania): geochemical evidence

Meteoric diagenesis of Upper Cretaceous and Paleocene-Eocene shallow-water carbonates in the Kruja Platform (Albania): geochemical evidenceIn the central part of the Kruja Platform (Albania) located in the Apulian passive margin, geochemical analyses (calcimetry, Sr, REE and isotopic, δ13C and δ18O) coupled with sedimentological and sequence stratigraphic study were carried out on Upper Cretaceous (CsB4, CsB5, CsB6 Biozones) and Paleocene to Middle Eocene shallow-water carbonates that crop out in the Kruje-Dajt massif (L'Escalier section) and Makareshi massif (La Route section). The lower values in Sr contents, the homogeneous δ18O values in both sections and the covariance between δ13C and δ18O values (La Route section) are attributed to diagenesis influence by a meteoric water-buffer system, supported by petrographic observations. Moreover, a new exposure surface during the Late Cretaceous time (between CsB5 and CsB6 Biozones) may be proposed according to the low or negative excursions of Sr values, the negative excursions of isotopic values in both sections and a positive peak of normalized REE values (La Route section). These variations correlate with the geochemical signal reported by the decreasing strontium isotope values of rudist shells in the Island of Brač carbonate platform (Apulia domain) during the late Middle Campanian (77.3 Ma). Also, this continental exposure is consistent with the global sea-level fall reported from the Boreal Realm, North Atlantic, and the southern Tethyan margin. This geochemical evidence is a complementary tool for the sedimentological analysis and suggests a maximum regression (a sea-level fall) at the transition between the CsB5 and CsB6 Biozones. The high values of Sr content in Middle Eocene carbonates (L'Escalier section) reflect changes in depositional environment from restricted to open marine conditions. REE values increase through transgressive systems tract, characterized by small increase of detrital input. However, anomalies of certain values in both sections suggest disturbances linked either to the changes in clay input and to diagenetic modifications. Peaks in dolomite content are linked with regressive episodes or tendencies, and dolomitic facies, as indicated by intertidal-supratidal depositional environments.

Multiwell Case Study - Increasing Oil Production in a Mature Field With Rigless Intervention

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper IPTC 11644, "Increasing Oil Production in a Mature Field by Rigless Intervention-A Multiwell Case Study," by Vivek Garg, ONGC, and Shubhranshu Ashesh (now with Shell), SPE, Kapil Seth, SPE, and Amit Govil, SPE, Schlumberger, prepared for the 2007 International Petroleum Technology Conference, Dubai, 4-6 December. The paper has not been peer reviewed. Reviving production from brownfields is a major focus for oil and gas companies. Workflow and procedures are outlined that were adopted to increase oil production from seven wells by use of rigless intervention on an offshore platform off the west coast of India. Introduction The field was discovered in 1987 and was put on production in 1994. The main reservoir is a Middle Eocene carbonate deposit characterized by vugs, dissolution channels, and fractures (both vertical and horizontal). Hence, high-permeability streaks provide conduits for early water breakthrough, which has been primarily responsible for the decline in oil production. The average water cut in the field is 80%. The offshore platform posed a special challenge because of its proximity to two water-injector platforms. Injection-water accumulation in the drainage zone covered by the wells of this platform was determined by current temperature anomalies and salinity contrast between the formation water and the accumulated injection water in the vicinity of the wells. The challenge was to study the flow dynamics so that additional perforations in the bypassed section would yield water-free oil. Method The strategy included well diagnostics followed by workover intervention to increase oil production and reduce the water cut. Production logging identified the flow profile. Then, pulsed-neutron logging was performed where required to determine hydrocarbon (HC) saturation in the cased-hole environment. Integration of data from openhole logs, production logs, and pulsed-neutron spectroscopy helped identify depleted zones and zones of bypassed oil. Through-tubing isolation plugs were placed to isolate water-producing intervals. Oil in bypassed zones was accessed with additional perforations. Formation-water salinity was calculated from pulsed-neutron logging to identify zones of injection-water breakthrough. Such intervals were avoided when adding new perforations. Seven intervened wells yielded a 53% increase in oil production to 2,708 BOPD. The average water cut of the platform decreased from 80 to 76%. Candidate Selection The objective of the operation was to increase oil production while minimizing water and gas production. Because no rig was available, operations had to be carried out with a modular skid unit, which constrained available tools and methods.

Applications of nummulitids and other larger benthic foraminifera in depositional environment and sequence stratigraphy: an example from the Eocene deposits in Zagros Basin, SW Iran

The Tale-Zang Formation in Zagros Mountains (south-west Iran) is a Lower to Middle Eocene carbonate sequence. Carbonate sequences of the Tale-Zang Formation consist mainly of large benthic foraminifera (e.g. Nummulites and Alveolina), along with other skeletal and non-skeletal components. Water depth during deposition of the formation was determined based on the variation and types of benthic foraminifera, and other components in different facies. Microfacies analysis led to the recognition of ten microfacies that are related to four facies belts such as tidal flat, lagoon, shoal and open marine. An absence of turbidite deposits, reefal facies, gradual facies changes and widespread tidal flat deposits indicate that the Tale-Zang Formation was deposited in a carbonate ramp environment. Due to the great diversity and abundance of larger benthic foraminifera, this carbonate ramp is referred to as a “foraminifera-dominated carbonate ramp system”. Based on the field observations, microfacies analysis and sequence stratigraphic studies, three third-order sequences in the Langar type section and one third-order sequence in the Kialo section were identified. These depositional sequences have been separated by both type-1 and type-2 sequence boundaries. The transgressive systems tracts of sequences show a gradual upward increase in perforate foraminifera, whereas the highstand systems tracts of sequences contain predominantly imperforate foraminifera.

Calcareous nannofossil biostratigraphic framework for middle Eocene sediments from ODP Hole 1260A, Demerara Rise

Ocean Drilling Program (ODP) Site 1260 recovered a near-continuous and expanded (187-m thick) middle Eocene carbonate sequence at mid-bathyal depths on Demerara Rise off Suriname, South America. A calcareous nannofossil biostratigraphic framework has been established for the sequence to aid multi-proxy cyclostratigraphic analyses. The diversity of calcareous nannofossils is reasonably high and preservation moderate throughout most of the section, and the proximity to the continent is indicated by the occurrence of braarudosphaerids in the lower three quarters of the section. The species richness of sphenoliths is particularly high, and our data confirm the occurrence of new sphenoliths during the middle Eocene as shown recently by Bown and Dunkley Jones (2006). In particular we corroborate the occurrences of Sphenolithus strigosus and S. runus that these authors found in their Zone NP16 Tanzania sediments, however we extend the ranges of these taxa down into Zone NP15 (CP13). Moreover we document the occurrence of Bramletteius serraculoides in Zone NP15 (CP13b) and the occurrence of Sphenolithus predistentus in NP16 (CP14a), extending their known ranges down column.

Dolomitization of Middle Eocene carbonate rocks, Abu Roash area, Egypt

Dolomitization is the most striking diagenetic process affecting the late Middle Eocene shelly limestone beds at Abu Roash and its neighboring areas. Petrographic investigations reveal that these rocks represent shelly-wackstone to packstone microfacies, that they were deposited probably within a shallow subtidal setting. The rock matrix has been subjected to both selective and complete dolomitization and replaced by equigranular mosaics of fine-to medium-grained dolomite (crystalline planar-s dolomite texture). Meanwhile, the embedded shell fragments show high resistance to dolomitization and their original calcite composition is mostly preserved. The petrographic features of the dolomite texture suggest that these dolomites were formed under near surface and low temperature conditions.

Structural controls on the evolution of the Kutai Basin, East Kalimantan

The Kutai Basin formed in the middle Eocene as a result of extension linked to the opening of the Makassar Straits and Philippine Sea. Seismic profiles across the northern margin of the Kutai Basin show inverted middle Eocene half-graben oriented NNE–SSW and N–S. Field observations, geophysical data and computer modelling elucidate the evolution of one such inversion fold. NW–SE and NE–SW trending fractures and vein sets in the Cretaceous basement have been reactivated during the Tertiary. Offset of middle Eocene carbonate horizons and rapid syn-tectonic thickening of Upper Oligocene sediments on seismic sections indicate Late Oligocene extension on NW–SE trending en-echelon extensional faults. Early middle Miocene (N7–N8) inversion was concentrated on east-facing half-graben and asymmetric inversion anticlines are found on both northern and southern margins of the basin. Slicken-fibre measurements indicate a shortening direction oriented 290°–310°. NE–SW faults were reactivated with a dominantly dextral transpressional sense of displacement. Faults oriented NW–SE were reactivated with both sinistral and dextral senses of movement, leading to the offset of fold axes above basement faults. The presence of dominantly WNW vergent thrusts indicates likely compression from the ESE. Initial extension during the middle Eocene was accommodated on NNE–SSW, N–S and NE–SW trending faults. Renewed extension on NW–SE trending faults during the late Oligocene occurred under a different kinematic regime, indicating a rotation of the extension direction by between 45° and 90°. Miocene collisions with the margins of northern and eastern Sundaland triggered the punctuated inversion of the basin. Inversion was concentrated in the weak continental crust underlying both the Kutai Basin and various Tertiary basins in Sulawesi whereas the stronger oceanic crust, or attenuated continental crust, underlying the Makassar Straits, acted as a passive conduit for compressional stresses.

Palaeoenvironmental Interpretation of the Middle Eocene Carbonates of Lakhpat, Northwestern Kachchh

Palaeoenvironmental Interpretation of the Middle Eocene Carbonates of Lakhpat, Northwestern Kachchh

Characterization of dolomitic rocks from the coastal mixing zone of the Floridan aquifer, Florida, U.S.A.

Petrographic, XRD, SEM, cathodoluminescence, and microprobe analyses were conducted to characterize dolomitic strata from a core penetrating the coastal mixing zone of the Floridan aquifer. Lithologic, geochemical, and water chemistry data were used to identify diagenetic trends, including modern water/rock interaction. The depositional environment of these middle Eocene carbonate sediments of the Avon Park Formation was intertidal to supratidal. Sabkha conditions caused penecontemporaneous deposition of evaporites and accessory minerals. Present water chemistry indicates undersaturation for gypsum and its survival is attributed to sluggish groundwater circulation. Dolomite crystals were commonly found to be compositionally zoned with crystal centers generally calcium-rich and less stable than the more stoichiometric crystal rims. Three stages of dissolution were recognized from SEM studies of these crystals: (1) initial development of dissolution pits; (2) partial dissolution of the unstable crystal center; and (3) complete dissolution of the unstable crystal center leaving hollow dolomite. Present-day mixing zone water is saturated with respect to stoichiometric dolomite. Near-stoichiometric dolomite cement may be filling pores in this zone and non-stoichiometric dolomite may be dissolving. The instability of calcian dolomite in the mixing zone may be related to compositionally modulated crystal lattice defects. Pervasive dissolved crystal centers in the saline zone are believed to have been highly disordered penecontemporaneous dolomite. Stoichiometric dolomite does not appear to be precipitating in this zone because of the presence of competing ions. Stable carbon and oxygen isotopic values for dolomite are comparable to those reported for Plio-Pleistocene dolomites. 14C activity implies some relatively young dolomite exists, probably as rims on older crystals.

Intra-slope canyon morphology and its modification by rockfall processes, U.S. Atlantic continental margin

Side-scan sonar data have been acquired at three different range settings and scales for a section of the continental slope. The data allow new perspectives on canyon morphology at varying complementary resolutions. Both regional context and site-specific details are provided. Intra-slope canyons on the lower slope are associated with outcrops of Middle Eocene carbonate rocks. Canyon geometries are heavily influenced by structural patterns of intersecting joints. Drop core analyses of surface sediments suggest that intra-slope canyon formation is not a geologically recent event, but occurred before 50,000 yrs B.P. Within the canyons, rocky debris results from episodic rockfall activity from the steep canyon walls. Dating of canyon floor sediments with inclusions of rockfall debris at one location suggests the last discrete period of rockfall there began about 10,000 yrs B.P. and ended about 6000 yrs ago. The rockfall activity may have coincided with a period of increased submarine dissolution along the margin that accompanied the upslope migration of the Western Boundary Undercurrent. Elements of the lower slope terrain may reflect surface solutional degradation of the joint patterns associated with the Eocene outcrops over longer time periods.

Petrology of Lower and Middle Eocene Carbonate Rocks, Floridan Aquifer, Central Florida: ABSTRACT

ABSTRACT Study of cores from a U.S. Geological Survey test well near Polk City indicates that the Avon Park-Lake City (Claibornian) and Oldsmar (Sabinian) Formations, which comprise most of the Floridan aquifer in central Florida, can be divided into six microfacies: foraminiferal mudstone; foraminiferal wackestone-packstone; foraminiferal grainstone; nodular anhydrite; laminated dolomicrite; and replacement dolomite. Dolomite containing variable amounts of nodular anhydrite forms more than 90 percent of the Avon Park-Lake City interval, whereas the Oldsmar is chiefly limestone. The depositional model inferred for these units is a broad, shallow-water marine platform with environments ranging from supratidal-sabkha to shallow-water shelf. Diagenetic pathways vary with rock type, but generally include: (1) marine phreatic -- grain micritization, and radially fibrous cementation within foraminiferal tests; (2) meteoric vadose -- minor leaching of aragonitic grains; and (3) meteroic phreatic -- neomorphism of unstable grains, dissolution of aragonitic allochems, formation of isopachous equant calcite cement and coarse spar in grainstones, and syntaxial calcite overgrowths on echinoderms. Several episodes of dolomite formation are recognized. Laminated dolomicrite formed syngenetically in a supratidalsabkha environment. Crystalline dolomite with nodular anhydrite formed early by replacement of limestone through reflux of dense, Mg-rich brines. Replacement dolomite not associated with evaporites and containing limpid crystals probably formed later by a mixed-water process in the subsurface environment. Late diagenetic processes affecting crystalline dolomites include hydration of anyhdrite to gypsum, partial dissolution of gypsum, alteration of gypsum to calcite (minor), and dissolution of calcian dolomite cores in stoichiometric crystals. Crystalline dolomite and grainstone are the only rock types that have high enough porosities and permeabilities to provide significant yields of water. Medium and finely crystalline dolomites show best values of porosity and permeability because they have high percentages of intercrystal and moldic pores that are well connected. Filling of pores by anhydrite or gypsum can significantly reduce porosity and permeability.

Diagenesis of Middle and Upper Eocene Carbonate Shoreline Sequences, Central Florida

Petrographic analysis of 14 cores penetrating the Inglis and Avon Park Formations (Floridan aquifer) reveals several distinct lithofacies representing multiple regressive-transgressive cycles, aerial exposure, and diagenesis. Evolution of pore spaces varies with the lithofacies and is a function of primary deposition and diagenetic history. The general tendency has been toward obliteration of primary voids by compaction and calcite and dolomite infilling, but several zones show dissolution enlargement. There is evidence for penecontemporaneous dolomitization in a supratidal environment, as well as post-depositional dolomitization by Mg-rich ground-water flushing. Various dolomite textures can be used to predict the environment of dolomitzation. These textures are (1) replacement dolomitization of the total sediment so that original fabrics are preserved; (2) dolomitization by aggrading neomorphic processes; and (3) replacement dolomitization of fossils and pellets only. Other diagenetic processes which have affected the lithofacies include dissolution, recrystallization and inversion, and several phases of calcite cementation. The hydrologic regime and the original texture of the carbonate rock controlled the types of cement present. Scalenohedral crystals have been precipitated from marine waters along the test walls of internal chambers of fossils. Small, equant, blocky crystals have precipitated from meniscus fluid in the vadose zone of ground water as coatings along internal chamber walls of fossils and at the contacts between grains. Large rhombohedral crystals have precipitated from fresh water of the phreatic zone as partial and complete infillings of intraparticle and interparticle spaces.