Cambro-Ordovician Sandstone Research Articles

Real-time porosity prediction using gas-while-drilling data and machine learning with reservoir associated gas: Case study for Hassi Messaoud field, Algeria

A novel inverse intelligent model is developed to predict porosity in real time while drilling. It applies established machine learning (ML) models to gas-while-drilling (GWD) data calibrated with core porosity measurements for a saturated oil reservoir. The data are associated with the Cambro-Ordovician sandstone reservoir in the giant Hassi Messaoud oil and associated gas field, located in central-eastern Algeria. The ML methods applied to the input dataset of nine variables recorded while drilling are, extreme learning machine (ELM), support vector regression (SVR), random forest regression (RFR), multilayer perceptron neural network (MLPNN), and generalized regression neural network (GRNN) and their results compared with a suite of statistical prediction accuracy performance metrics: correlation coefficient (R), Nash-Sutcliffe efficiency coefficient (NSE), the root mean square error (RMSE), and the mean absolute error (MAE). The ELM model was found to deliver the highest accuracy in predicting real-time porosity for the validation data records (R∼0.929, NSE∼0.865, RMSE∼3.325%; MAE∼ 2.595%). The method and results identify a rapid and accurate new approach to the real-time prediction of rock porosity with only GWD data. The GWD input variables are typically available in every field well drilled at a much lower cost than running measurement while drilling (MWD) or wireline logging tool traditional used to provide porosity information. Each of the evaluated ML models could be easily integrated with real-time monitoring systems as wells are drilled. By doing so, substantial time saving and economic savings could be achieved in comparison with the extensive laboratory measurements and MWD/wireline tools that currently required to assesses reservoir porosity in field development wells. The method also provides petroleum engineers with real-time “quick look” porosity information during drilling that can contribute to safe real-time decision making. The method has potential for gas and oil reservoir evaluations.

Provenance of the great Cambrian sandstone succession of northern Gondwana unravelled by strontium, neodymium and lead isotopes of feldspars and clays

AbstractThe great Cambro–Ordovician sandstone succession of northern Gondwana is one of the most voluminous siliciclastic deposits on Earth. Its deposition took place following the Neoproterozoic Pan‐African Orogeny and the consolidation of Gondwana, a crucial episode in Earth history. Its provenance is of great importance for reconstructing palaeogeographic and tectonic settings and determining sediment dispersal patterns. In Israel and its environs, Palaeozoic sandstones overlie the juvenile Neoproterozoic basement of the Arabian–Nubian Shield. These basement rocks were considered as an important sediment source to the Cambro–Ordovician succession. However, recent studies established that a dominant portion of detrital zircons in the succession was derived from Archean–Proterozoic remobilized terranes, suggesting transportation distances of thousands of kilometres beyond the Arabian–Nubian Shield, and thus question the provenance of other, less weathering‐resistant constituents, which were not necessarily derived from the same localities. Feldspars and clays were separated from 13 samples and analyzed for mineralogical and geochemical compositions, strontium, neodymium and lead isotopes, in order to further constrain the provenance of the Cambro–Ordovician succession of northern Gondwana. Detrital feldspars have εNd values significantly different from feldspars of the Arabian–Nubian Shield, ranging from −8 to −14, with model ages of 950 to 1750 Ma, more radiogenic lead compositions, and μ (238U/204Pb) values ranging from 9·8 to 10·26. Clays are dominated by illite, smectite and kaolinite, with εNd values ranging between −14 and −9, with 87Sr/86Sr ratios up to 0·757. These findings point to a significant contribution of remote ancient terranes from outside the Arabian–Nubian Shield to the Cambro–Ordovician sandstone of northern Gondwana. The possible sources for these sediments are the Saharan Metacraton and the southern regions of the East African Orogen.

Detrital rutile U-Pb perspective on the origin of the great Cambro-Ordovician sandstone of North Gondwana and its linkage to orogeny

LA-ICPMS U-Pb geochronology of detrital rutile from North Gondwana Cambro-Ordovician sandstones is reported for the first time. Our sampling sites in Israel, Jordan and Ethiopia are situated thousands of kilometers apart thus providing a continental-scale perspective on the cooling and unroofing history of the provenance of the great Cambro-Ordovician sandstone sheet.Detrital rutiles in the Cambrian sandstone of Israel and Jordan define a unimodal age concentration at 0.59–0.58Ga, preceding the timing of major orogenic sutures and predating the final assembly of Gondwana. Cooling to below the rutile U-Pb closure temperature followed continental-scale Pan-African granitoid intrusion at 0.63–0.60Ga, signifying that rather than representing high-grade metamorphism the detrital rutile age spectra reflect widespread resetting, cooling and exhumation associated with late to post-tectonic igneous activity. The unimodal 0.59–0.58Ga age spectra do not change significantly up the Cambrian sequence, implying exhumation prior to Cambrian sedimentation generated ample crustal material with an almost uniform U-Pb rutile signature that was prone to deliver immense amounts of detritus. Therefore, rather than reflecting progressive erosion of Cambrian Supermountains, the North Gondwana Cambrian sandstone represents secondary denudation of an already-exhumed metamorphic basement.Alongside the older (0.59–0.58Ga) detrital rutile population, which dominates the Cambrian sequence, 0.55Ga detrital rutiles first appear in the Ordovician of Jordan while 0.54Ga rutile locally dominates the Ordovician from Ethiopia, some 2000km upstream the Cambro-Ordovician fluvial system, close to its headwaters. The Ordovician drainage basin has thus been extended to include crustal vestiges that were exhumed coeval with Gondwana consolidation, but it is likely that most of the rock carapace that has been eroded from consolidating Cambrian orogenic sutures has been delivered to the south (modern coordinates).

The dispersal of the Gondwana Super-fan System in the eastern Mediterranean: New insights from detrital zircon geochronology

We report here new LA-ICPMS detrital zircon U–Pb ages of a quartzite from the autochthon of Peloponnesus (Feneos locality), southern Greece. The rock classifies as a mature quartz arenite and belongs to an original shale–sandstone succession now metamorphosed into a phyllite–quartzite unit. Zircon age clusters at 0.52–0.75, 0.85, 0.95–1.1, 1.75–2 and 2.4–3Ga point at the Saharan Metacraton and the Transgondwanan Supermountain as contributing sources; the youngest concordant grain is 522Ma old. Our data collectively suggest deposition during the Cambro-Ordovician in a collisional setting and are in excellent agreement with those of the virtually intact Cambro-Ordovician sandstone–shale sequences of Libya (Murzuq and Kufrah basins) and the Middle East (Israel and Jordan), interpreted to have been deposited in the Gondwana Super-fan System which draped the northern Gondwanan periphery from ~525 to 460Ma. By contrast, re-evaluating the available zircon age-distribution pattern and depositional setting of an analogous sequence forming the autochthon of north-central Crete (Galinos beds) we demonstrate that it was originally deposited in a completely different setting, i.e. in an accretionary/fore-arc complex outboard of the south Laurussian active margin (Pelagonia) during the Late Carboniferous. Comparing similar Cambro-Ordovician metasiliciclastic rocks from north-eastern Crete (Sfaka paragneiss), north-central continental Greece (Vertiskos terrane), north-western Turkey (central Sakarya terrane) and the Romanian Carpathians we show that their detrital zircon distribution patterns testify to an original depositional setting similar to that of Peloponnesus (Feneos), Libya and the Middle East. Using key time-frames from previously published palaeogeographic reconstruction models we are able to trace in space and time the Palaeozoic–Early Mesozoic wondering paths of the aforementioned sequences. Thus, time- and facies-equivalent rocks presently cropping out in the eastern Mediterranean share a common provenance from the Gondwana Super-fan System which was diachronously dispersed between Early Silurian and Early Triassic.

Hydrogeology of the Saq Aquifer Northwest of Tabuk, Northern Saudi Arabia

Groundwater resources are precious commodity in general and for arid regions in particular. Their exploration, exploitation, protection and management play significant role in societal activities. In the Kingdom of Saudi Arabia, extensive aquifers are available in the Arabian Shelf. The Saq aquifer is one of the most important aquifers in the northwestern part of Saudi Arabia. It is made up of a Cambro-Ordovician sandstone formation, which extends over 1200 km in Saudi Arabia and northwards in Jordan. Groundwater occurs under unconfined conditions in the southwestern part and changes to confined conditions northward. The study area is located at about 50 km northwest of Tabuk, where many agricultural projects have been established depending on groundwater of the Saq aquifer for irrigation. The present study deals essentially with the hydrogeology of the Saq aquifer and the impact of extensive pumping on both groundwater levels and groundwater quality. Pumping and recovery tests were carried out in order to estimate the aquifer parameters. A total number of 192 groundwater samples were collected during the period 2000-2004 for quality investigation. Results of analysis of pumping test data showed that the average transmissivity and storativity are 1572 m 2 /day and 2.86 × 10 -4 , respectively. Values of average TDS of groundwater range between 540 and 670 mg/l, which indicate that it can be used safely for irrigation. The TDS values showed general decrease towards the northeastern part of the study area, in the direction of flow. Extensive pumping from 260 deep wells with an average discharge rate of 0.015 m³/sec has resulted in continuous groundwater level drop since 1999 with an average rate ranges between 2.3 − 10.5 m/year. Additionally, the effects of pumping on the water quality for the period 2000-2004 were studied

Mass-production of Cambro–Ordovician quartz-rich sandstone as a consequence of chemical weathering of Pan-African terranes: Environmental implications

A vast sheet of mature quartz sand blanketed north Africa and Arabia from the Atlantic coast to the Persian Gulf in Cambro–Ordovician times. U–Pb geochronology of a representative section of Cambrian sandstone in southern Israel shows that these sediments are dominated by 550–650 Ma detrital zircons derived from Neoproterozoic Pan-African basement. The short time lag between magmatic consolidation of a Pan-African source and deposition of its erosional products indicates that, despite their significant mineralogical maturity, the voluminous quartz-rich sandstones on the northern margin of Gondwana are essentially first-cycle sediments. Mass production of these voluminous first-cycle quartz-rich sandstones resulted from widespread chemical weathering of the Pan-African continental basement. We suggest that conditions favoring silicate weathering, particularly a warm and humid climate, low relief and low sedimentation rates prevailed over large tracts of Gondwana in the aftermath of the Pan-African orogeny. An unusually corrosive Cambro–Ordovician atmosphere and humid climate enhanced chemical weathering on the vegetation-free landscape. We infer that late Neoproterozoic–Cambro–Ordovician atmospheric pCO2 rose as a consequence of widespread late Neoproterozoic volcanism, followed by an uptake of CO2 by chemical weathering to produce the Cambro–Ordovician sandstone as a negative feedback.

Pyrobitumen occurrence and formation in a Cambro–Ordovician sandstone reservoir, Fahud Salt Basin, North Oman

The Cambro–Ordovician Barik Sandstone reservoirs in the Fahud Salt Basin in Oman contain bitumen which may fill up to 40% of the porosity. In well Jaleel-1, this bitumen was isolated (according to kerogen procedure) and typed by NMR, elemental analysis and density measurements. The isolated bitumen is characterized by: (1) a highly aromatic character (NMR 75% C Aro, H/C atomic ratio: 0.65), (2) a very high sulphur content (4.2%) and (3) a relatively high density (1.3–1.4 g/cm 3). The insolubility and the reflectivity of the bitumen (1.2% Vr) qualify it as a low mature pyrobitumen. The combination of Rock-Eval and density data was used to calculate the actual volume of the pyrobitumen in the rock, as a percentage of porosity. It was found that the pyrobitumen volume shows a negative correlation with total porosity, indicating that small pores are more invaded by bitumen than larger ones. Finally, closed system pyrolysis experiments, performed on oils with different NSO contents, indicate that an in situ oil with a very high content of NSO compounds is required to generate such large amounts of pyrobitumen in the pore system. These observations suggest that the precursor oil of the current pyrobitumen was a very heavy oil tentatively assumed to be the result of a severe biodegradation. Basin modeling shows that the reservoir was charged already in Devonian times. A major uplift brought the oil accumulation near the surface during the Carboniferous and a rather regular burial to the present day position (4500 m, 140°C) (Loosveld et al., 1996). This scenario, involving a residence time at shallow depth, strengthens the biodegradation hypothesis. The numerical modeling, which involves the IFP kinetic model for secondary oil cracking, suggests that pyrobitumen formation is a very recent event. Inclusion of pyrobitumen particles within quartz overgrowth, containing fluid inclusions, provides an upper temperature limit for the beginning of pyrobitumen formation which comforts the result of kinetic modelling.

Groundwater quality in the Saq aquifer, Saudi Arabia

The Saq aquifer is a Cambro-Ordovician Sandstone Formation that extends over 1200 km in Saudi Arabia and northwards in Jordan. It is an enormous groundwater reservoir where water is found under both unconfined and confined conditions. The main concern of this study was the quality of the Saq aquifer groundwater. This includes a description of the chemical composition of the groundwater, the spatial variation of its salinity and the processes that control this variability. A total of 384 wells drilled in the Saq aquifer were sampled and water analyses performed for the major ionic composition. The data were analysed using software computer programs. Saturation indices were calculated, ionic relationships were studied, hydrochemical facies were determined and water types were identified. The principal processes that control the groundwater quality variations are also discussed. The results of this study provide information needed for groundwater quality management in the Saq aquifer.

The hydrogeology and groundwater resources development of the Cambro-Ordovician sandstone aquifer in Saudi Arabia and Jordan

The Saq Cambro-Ordovician sandstones form the major national aquifer in Saudi Arabia and Jordan. Significant abstraction of groundwater has taken place since the early 1950s and monitoring of the aquifer responses indicates a moderate transmissivity but high specific yield. The groundwater quality is excellent for irrigation which is the present major use, and is also good for potable purposes. Although large drawdowns are being experienced locally in confined areas of the aquifer the unconfined area responses show the aquifer to have an excellent potential for development under groundwater mining principles that may be considered for use outside the areas of occurrence.

Diagnosis of marine and non-marine environments from the Cambro-Ordovician sandstones of Jordan

The Precambrian shields of the Sahara and Arabia are overlain by a thick laterally extensive Cambro-Ordovician sandstone blanket. This is generally divisible into a lower coarse pebbly cross-bedded facies overlain by a finer-grained facies with a characteristic suite of trace fossils. This paper describes the sedimentology of the Gambro-Ordovician sequence where it crops out in the Southern Desert of Jordan. The lower coarse pebbly facies consists of a complex of channels up to 300 m wide and 15 m deep with occasional shale-filled channels of similar dimensions. Three formations of this facies occur. The lowest overlies a mature pediment with residual inselbergs cut in the Precambrian basement. The succeeding two formations have channelled bases. This facies is attributed to three cycles of pedimentation and sedimentation on braided alluvial outwash plains. The overlying facies consists of an alternation of medium grained well sorted sandstones and rare shales. The sandstones are devoid of channelling being composed of laterally extensive tabular-planar cross-bedding. The interbedded shales have sheet geometries, in contrast to the shoestring shaped shales of the facies beneath. They consist of laminated silts and microcross-laminated fine sands with a diverse suite of trace fossils. This upper facies is attributed to deposition on a marine shelf by the alternation of migrating sand shoals and tidal flats.

Occurrence of Pure, Well-Crystallized 1m Illite in Cambro-Ordovician Sandstone from Rhourde El Baguel Field, Algeria

ABSTRACT Well-crystallized, relatively pure 1 M illite occurs in clay fractions of Cambro-Ordovician sandstones from the Sinclair Mediterranean Petroleum Company Rhourde el Baguel No. 1 well, Algeria. This is the first report of such a clay (well-crystallized 1M, relatively pure) from a sedimentary rock. Preliminary data show that the chemical composition of this clay approaches that of ideal muscovite. Petrographic evidence and the lack of a terrigenous source for 1M illite strongly indicate a diagenetic origin. It is suggested that this clay is the result of post-depositional, in-place alteration of feldspars in the presence of saline formation waters.