Tahe Area Research Articles

New insights into the carbonate karstic fault system and reservoir formation in the Southern Tahe area of the Tarim Basin

As worldwide hydrocarbon exploration has extended from shallowly to deeply buried strata, reservoir quality has attracted substantial and persistent interest in petroleum geology. In particular, deeply buried strata (>5500 m) in the Tarim Basin have attracted considerable attention because carbonate reservoirs that have experienced fracture or dissolution have also been shown to demonstrate considerable hydrocarbon potential. Therefore, it is necessary to determine how these reservoirs are developed and distributed in detail from both scientific and practical standpoints.In this paper, we address this issue using a case study in the southern Tahe area, which is contained within the largest Palaeozoic marine oilfield in China. In the northern Tahe area, mega-paleokarst systems developed in the Ordovician strata; however, the reservoir quality in the southern part of the Tahe area is relatively poor because it is covered by insoluble formations during karstification. Observations of cores and analyses of images of well logging demonstrate that these reservoirs are dominated by caves, vugs and fractures that have developed near faults. We speculate that the faults penetrating insoluble formations represent the main dissolution passages that originally developed these karstic fault systems. Additionally, we analyse a series of outcrops, seismic data, and structures to characterize the spatial geometry of these major faults and their surrounding fractures in detail. Most of these are strike-slip faults, and their subsequent reservoirs can be divided into three categories based on their development, including dendritic, sandwich and slab reservoirs. Recent studies demonstrate that karstic fault reservoirs are most common traps in the study area. Although various types of carbonate karstic fault reservoirs are represented in this region, the dendritic karstic fault reservoir is the most hydrocarbon-rich.Guided by these initial results, 108 wells were drilled from 2013 to 2014, producing 485 thousand tons of oil and yielding success ratios greater than 89%. The average production of dendritic reservoirs is 37.4 tons per day (t/d), while those of sandwich and slab types are 20.2 t/d and 14.0 t/d, respectively. These results represent significant references for future hydrocarbon exploration and the development of similar deeply buried karstic fault reservoirs in the Tarim Basin and elsewhere.

Hydrothermal dissolution of Ordovician carbonates rocks and its dissolution mechanism in Tarim Basin, China

The dissolution of carbonate rocks has a significant control on reservoir quality under deep burial conditions, which has become a focus of studies in recent years. A series of hydrothermal minerals and combinations, such as fluorite, dickite, barite and hydrothermal dolomite, were discovered in the Ordovician strata in some exploration wells in the Tahe area. Moreover, the analysis results of carbon and oxygen isotope, REE, strontium isotope and inclusion homogenization temperature further confirmed that hydrothermal fluids existed in the Ordovician strata of Tahe areas. In this paper, the dissolution mechanism of carbonate rocks under deep burial conditions is discussed. It was considered that the hydrothermal dissolution required three conditions, i.e., the development of fault system, the presence of acidic fluids and a fall in temperature. The major theoretical foundation was the retrograde solubility of carbonate rocks that could be achieved through temperature drop and the upward migration of hydrothermal fluids. The dissolution of carbonate rocks caused by hydrothermal fluids normally occurred in the vicinity of migration pathways (faults or other pathways), which could be identified as good reservoir development zones.

Discovery and basic characteristics of high-quality source rocks found in the Yuertusi Formation of the Cambrian in Tarim Basin, China

The Upper Paleozoic strata of the Tarim Basin have abundant resources of marine oil and gas. In the Tahe area, Halahatang area, and Tazhong area of the basin, many large-scale oilfields have been found. These oilfields have a confirmed oil and gas reserves worth more than 2.5 billion tons and have completed the annual output of more than 14 million tons of marine oil and gas equivalent. The belief that the only main hydrocarbon source rocks are of the Cambrian or Ordovician is still controversial. Chemists have made significant progress and have effectively lead the oil and gas exploration in Tarim Basin. Due to the complexity of the basin and the limitation of samples, the research work, and fine contrast is restricted. In this article, we investigated the Cambrian strata outcrop of Tarim Basin in detail. By analyzing a lot of outcrops, high-quality hydrocarbon source rocks of Yuertusi Formation have been found in more than 10 outcrop points in Aksu region. The source rocks' lithology is black shale with total organic carbon (TOC) content that ranges between 2% and 16%. Total organic carbon (TOC) of the black shale layer could be as much as 4%–16%, especially in the outcrops of the Yutixi and Shiairike. This by far is the best marine hydrocarbon source rock that was found in China. The source rocks were distributed consistently in the Aksu region, the thickness of which is about 10–15 m. It was formed in a sedimentary environment of a middle gentle slope to a low gentle slope. Organic matter enrichment is controlled by the upwelling currents. The thick strata of dolostone that developed in the Xiaoerblak Formation are considered to be good reservoirs of the beach and microbial reef in the upper strata of Yuertusi Formation. No hydrocarbon source rocks have been found in the outcrop of Xiaoerblak Formation. The thick strata of gyprock and mudstone development are a set of satisfactory cap layer in the Lower Cambrian. This hydrocarbon accumulation combination has great exploration potential.

Multi-layered ordovician paleokarst reservoir detection and spatial delineation: A case study in the Tahe Oilfield, Tarim Basin, Western China

Paleokarst systems are major carbonate reservoirs that can form significant and very large oilfields. The Ordovician carbonate reservoirs of the Tahe Oilfield represent a special type of hydrocarbon reservoir consisting primarily of paleokarst caves and fractures. Detailed characterization of paleokarst reservoirs in the Tahe area is challenging because of their heterogeneous distribution and, in particular, because these reservoirs are at depths exceeding 5500 m.This paper describes a detailed approach for integrating data from cores, imaging and conventional well logs, and high-quality seismic datasets to delineate the complex paleokarst systems in the Tahe Oilfield, particularly the paleocaves in the run-off zone. Based on the hydrologic genetic relationships between the karst and paleocaves observed in outcrops, we divided the paleocaves in the run-off zone into chambers, main channels, and branch channels. The chambers, which are generally taller than 15 m, are generally collapsed. The main channels, mostly 5–15 m tall, were the main passages for underground water flow and display good connectivity. The branch channels, typically less than 5 m tall, are relatively dispersed and still have a considerable volume.This method was applied to the area around Well T615, and two layers of paleocaves in the run-off zone were interpreted. Seismic mapping and three-dimensional modeling of the paleocaves were performed, and the lengths, widths, areas and volumes of these three types of paleocaves were statistically analyzed. Although most of the chambers have been penetrated by existing wells, the areas where the main channels and a dense network of branch channels developed are favorable targets for future drilling in the Tahe Oilfield. Our method of detecting and analyzing the paleokarst reservoir structure is applicable to similar paleokarst reservoirs.

Dissolution-filling mechanism of atmospheric precipitation controlled by both thermodynamics and kinetics

Affected by structural uplift, the Ordovician carbonate rockbed in the Tarim Basin, China, was exposed to dissolution and reformation of atmospheric precipitation many times, and formed a large quantity of karst caves serving as hydrocarbon reservoir. However, drilling in Tahe area showed that many large karst caves, small pores and fractures are filled by calcite, resulting in decrease in their reservoir ability. Calcite filled in the karst caves has very light oxygen isotopic composition and 87Sr/86Sr ratio. Its δ18OPDB ranges from −21.2‰ to 13.3‰with the average of −16.3‰ and its 87Sr/86Sr ratio ranges from 0.709561 to 0.710070 with the average of 0.709843. The isotope composition showed that calcite is related to atmospheric precipitation. Theoretic analyses indicated that the dissolving and filling actions of the precipitation on carbonate rocks are controlled by both thermodynamic and kinetic mechanisms. Among them, the thermodynamic factor determines that the precipitation during its flow from the earth surface downward plays important roles on carbonate rocks from dissolution to saturation, further sedimentation, and finally filling. In other words, the depth of the karstification development is not unrestricted, but limited by the precipitation beneath the earth surface. On the other hand, the kinetic factor controls the intensity, depth, and breadth of the karstification development, that is, the karstification is also affected by topographic, geomorphologic, climatic factors, the degree of fracture or fault, etc. Therefore, subject to their joint effects, the karstification of the precipitation on the Ordovician carbonate rocks occurs only within a certain depth (most about 200 m) under the unconformity surface, deeper than which carbonate minerals begin to sedimentate and fill the karst caves that were formed previously.

The Early Hercynian paleo-karstification in the Block 12 of Tahe oilfield, northern Tarim Basin, China

It is indicated in this paper that the karstification occurred in Block 12 of Tahe oilfield during Early Hercynian stage on the basis of identification features obtained through the latest research on seismology, petrology, and geochemistry. Distinguishable markers of the paleo-karstification mainly include: the lost strata and unconformable contact occurring between Lower–Middle Ordovician carbonates and Lower Carboniferous Bachu Formation, the occurrence of collapse breccias and fillings in subsurface stream, the evident difference between cave-fill calcite and matrix carbonate in the Sr/Ba ratio, and the log response for cavernous formations. The paleo-geomorphology of Early Hercynian karstification can be divided into four units: karst highland, I- and II-level karst slope, and karst basin in Tahe area, and the Block 12 was basically located in II-level karst slope. Three karst cycles developed in the Block 12 according to Early Hercynian tectonism characterized by multiphase pulse-type uplift movement and three sets of cavernous formation formed. II- and III-level cavernous formations mainly developed in Block 12. The Early Hercynian karstification is distributed on the north area of the pinch-out line of Upper Ordovician Sangtamu Formation in the plane, and is vertically developed in the Lower–Middle Ordovician carbonates. The karstification is essentially controlled by the paleo-geomorphology and fault. Fissures and caves formed by the karstification are the keys to affecting reservoir quality. The evolution model of Early Hercynian karstification has been established on the basis of the synthesis of the above-mentioned research results. It can be divided into three stages: rapidly uplifting and denudation stage, three episodes of uplifting and karstification, and subsiding and reworking stage.

Fractured - Vuggy Genesis Model of Middle - Lower Ordovician Carbonate Rocks in Tahe Oilfield of Trim Basin, Western China

The fracturedvuggy characteristics of carbonate rocks of Middle Lower Ordovician limestone in Tahe Oilfield was studied in the article. By analysis of the affecting factors of reservoir space include sedimentary diagenesis, tectonism and karstification, we found deposition determined the Formation and the number of primary porosity, diagenesis affected the pore destruction and construction, tectonism controled the reservoir burial history and the Formation of structure cracks and weathering fissure, karstification affected the latter change of the previous diagenetic porosity and remodeled the large hole-cave reservoir in Tahe Oilfield. Then the fissure-cave porosity evolution is studied. And we found that the Lower or dovician carbonate rocks in Tahe area experienced shallow marine diagenetic environment, shallow surface meteoric diagenetic environment, early shallow burial diagenetic environment, uplift exposed supergene meteoric diagenetic environment, shallow deep burial diagenetic environment. As a result, the Tahe Oilfield Ordovician fissure-cave porosity evolution model is established.

Application of sequence stratigraphy to Triassic terrestrial strata in Tahe area of Tarim Basin

Based on a synthetic geological study of drilling, well logging and seismic data, core observations and geochemical analysis, it is recognized that Triassic sedimentary facies in the Tahe area of Tarim Basin include braided river deposits filling erosional valleys, and sublacustrine fan, canyon and delta facies. Braided river deposits filling erosional valleys are dominated by coarse-grained lithic quartz sandstone with oblique bedding, and represent the most important sedimentation type of sandstone in the study area. Sublacustrine fan and canyon facies are mainly distributed in the Middle Oil Member. Most delta sediments are deposited in highstand system tract (HST). Because of frequent changes in base level, delta sediments are commonly eroded and rarely preserved. Sedimentary cycles are clearly reflected by lithology, sedimentary structures and well logging data, and are closely related to the changes in lacustrine level. In accordance with the basic principle of sequence subdivision, seven type-I boundaries can be recognized in Triassic strata and six type-I sequences are subdivided correspondingly. In general, lowstand system tract (LST) is well developed within stratigraphic sequences and forms the main body of reservoir sandstone in this area; highstand system tract (HST) and transgressive system tract (TST) are often eroded by upper sequences or missed. Although various factors have different influences on terrestrial sequence stratigraphy, the classical sequence stratigraphy theory proposed by VAIL can be applied to terrestrial strata.

Characteristics of Hydrocarbon Accumulation in the Paleozoic Donghetang Formation in the Southwestern Tahe Area

Abstract:The ancient structure characteristic, correlation of the oil and the hydrocarbon source rock characteristics, hydrocarbon migration trace, types and conditions of traps, migration passages and characteristic of hydrocarbon accumulation are researched in this paper. It is shown through the analysis that two main large tectonic activities after the Early Hercynian orogeny resulted in different tectonic patterns in the study area. Two main hydrocarbon infills occurred in the Donghetang Formation, the first occurred in the Early Hercynian resulting in the ancient hydrocarbon accumulation in the northern Tahe, the second infill was a large amount that occurred in places beneficial for hydrocarbon accumulation, such as structural traps and structural‐stratigraphic traps formed in the Early Himalayan orogeny after migration along the faults through source rocks and other passages. Before the earlier period of the Himalayan orogeny, the petroleum mainly migrated to the north, whereas petroleum migrated to the south and southeast because of the structural reverse in the Himalayan orogeny, so the middle and later period of the Himalayan orogeny is the key period for hydrocarbon accumulation. The model of “oil generation formed early, hydrocarbon accumulation controlled by the faults through source rocks and structures formed late” is proposed. It is pointed out that the south of the research area is currently the beneficial district for hydrocarbon accumulation, which provides the basis for future petroleum exploration.