Effective reservoirs and hydrocarbon phases in ... The volcanic reservoirs contain mostly weathered crust composed of dissolution . ... 4 CHINA PETROLEUM EXPLORATION Vol. 21, No. 1,

CHINA PETROLEUM EXPLORATION

Volume 21, Issue 1, January 2016

Received date: 31 Aug. 2015; Revised date: 27 Nov. 2015. Corresponding author. E-mail: [email protected] Foundation item: Copyright © 2016, Petroleum Industry Press PetroChina. All rights reserved.

Effective reservoirs and hydrocarbon phases in discordogenic structures in Mahu Sag and its periphery, Junggar Basin

Wei Yanzhao1, Chen Gangqiang2, Wang Feng2, Bian Baoli2, Wu Aicheng2, Yang Fan1, Yang Chun1, Yang Hui1

1. Research Institute of Petroleum Exploration and Development of PetroChina;

2. Exploration and Development Research Institute of PetroChina Xinjiang Oilfield Company

Abstract: In Junngar Basin, a series of large anticliens and faulted anticlinal structures are developed in Carboniferous and Lower

Permian in Mahu sag and its periphery. They are distributed at the center of of high-quality hydrocarbon kitchen of the Lower

Permian Fengcheng Formation and its perihery in Mahu sag, where the conditions of reservoir formation superior, thus they are

important exploration areas of deep oil and gas in Junggar Basin. Due to the deeper burial depth of these structures, however, the

exploration and development potential in these areas is mainly affected by the following two aspects. The first is whether effec-

tive reservoirs are developed in a large scale in the target layers. And the second is whether it is gas-phase hydrocarbon. It is indi-

cated from comprehensive analysis of aeromagnetic anomaly processing and interpretation, seismic horizon velocity analysis,

source rock evolution and natural gas genetic classification that volcanic weathering crust reservoirs are developed at Northern

Mahu anticline, Well Da 1 anticline and central-northern Mahu anticline. Based on evolution stages of Fengcheng Formation

source rocks, spatial locations of anticlinal structures and oil cracking depth, it is predicted that the hydrocarbon phase is kero-

gen-cracking gas in Northern Mahu and Mahu anticlines, oil-gas paragenesis in Well Da 1 anticline and dominantly gas in South-

ern Mahu anticline.

Key words: Junggar Basin, Mahu sag, discordogenic structure, effective reservoirs, hydrocarbon phase

The world’s major petroliferous basins are increasingly challenged by the difficulty in extracting oil and gas reser-ves found in the more thoroughly explored medium-shallow depth layers. With the advancement of hydrocarbon geolo-gic theories and drilling engineering techniques, deeper ex-ploration ventures have become the norm[1]. The recent dis-coveries of multiple medium-to-large reservoirs in the deep layers of the Tarim and Sichuan Basins have revealed the broad coverage and huge potential of hydrocarbon exploration and development in the onshore deep layers in China[27]. The Junggar Basin is one of the largest superimposed petro-liferous basins in western China and holds sizeable oil and gas resources. Its features of evolution and source-reservoir assemblage – “early hot and late cold” (geothermal gradient was 4.55.5C/100 m pre-Triassic and 1.93.5C/100 m post- Triassic) and “deep source and shallow reservoir” (major source rocks exist in Upper Paleozoic, and high-quality res-ervoirs in Mesozoic and Cenozoic), indicates the potential presence of abundant hydrocarbon resources deep within the basin[89]. In 2006, by drilling the well Moshen 1, a risk ex-ploration well, PetroChina discoverd the deep Mosuowan large structure in this basin, yet made no breakthrough as there are no effective reservoirs. Xinjiang Oilfield Company has recently carried out a new round of overall study on the Mahu Sag and its periphery, and reliably ascertained a batch

of anticline and faulted anticlinal traps of large areas in Carboniferous and Lower Permian. Located in or near the Permian hydrocarbon generation center of the Mahu Sag, these traps have superior hydrocarbon source condition and high potential of hydrocarbon exploration, making them key risk exploration targets[1011]. In an effort to determine the existence and whereabouts of large-scale effective reservoirs and the correlating hydrocarbon phases within the deep lay-ers, a thorough analysis was conducted. Despite the lack of drilling data, the combined utilization of aeromagnetic data, seismic velocity spectrum, hydrocarbon generation potential and evolution features of the source rocks and the geochemi-cal data of the discovered oil and gas sufficed to accurately predict the reservoirs and hydrocarbon phases of major anti-clinal traps in deep layers within the study area. The find-ings serve the secondary purpose of providing key informa-tion for deploying risk exploration in similar structures.

1. Geologic setting

The Junggar Basin, located at the north piedmont of the northern Tianshan Mountains, Xinjiang, is a late-Paleozoic (C2-P) to Mesozoic-Cenozoic superimposed basin which developed on the Pre-Cambrian crystalline basement and the Upper Paleozoic (D-C) fold basement. Significant quan-

2 CHINA PETROLEUM EXPLORATION Vol. 21, No. 1, 2016

tities of oil and gas have been found throughout it and range in origin from Carboniferous to Neogene[12]. The Junggar Basin includes six primary tectonic units: 1) the piedmont thrust belt of the Tianshan Mountains 2) the Central Depres-sion, 3) the Luliang Uplift, 4) the Wulungu Depression, 5) the Eastern Uplift and 6) the Western Uplift. The Mahu Sag, situated at the northwest edge of the Junggar Basin and northwest of the Central Depression, is adjacent to the western edge of the Luliang Uplift to the east and the West-ern Uplift to the west (Fig.1). It is one of the most important hydrocarbon-rich sags in the Junggar Basin and a large quantity of oil reserves have been proved in the faulted belt of its Western Uplift. Large amounts of oil and gas have also been discovered in Triassic layers in the northern slope of the Mahu Sag. Major source rocks of this sag exist in the Upper Paleozoic, including the Lower Permian Jiamuhe

Formation, Fengcheng Formation, and Middle Permian Lower Wuerhe Formation. In addition, Carboniferous source rocks may exist, however this remains to be proved. In re-cent years, a new overall study and fine structural mappings have been completed for the lower assemblage (C-P) in the deep layers in the Mahu Sag and its periphery, and a number of Carboniferous and Lower Permian structures have been identified. These structures are mostly anticlines and faulted anticlines. They feature multiple target layers, large trap ar-eas, and high certainty. The Northern Mahu Anticline and Mahu Anticline in the Mahu Sag, Well Da1 Anticline and Southern Mahu Anticline at the Dabasong Bulge, and the Permian hydrocarbon generation center and its periphery, have exceptionally better source conditions and large hy-drocarbon exploration potential. They are vital risk explora-tion targets in deep layers of the Junggar Basin (Fig.1).

Fig. 1 Location of structures in Mahu Sag and its periphery

2. Reservoirs in discordogenic structures in Mahu Sag and its periphery

Drilling data in the Mahu Sag and its periphery shows that there are three main types of reservoirs in deep Carbon-iferous and Permian formations: volcanic reservoir, dolo-mitic sandstone reservoir and clastic reservoir. The volcanic reservoirs occur principally in Carboniferous and the Lower Permian Jiamuhe Formation. The dolomitic sandstone res-

ervoirs exist in the Lower Permian Fengcheng Formation. The clastic reservoirs, primarily composed of glutenite, are developed in various members of Permian formations (Fig.2). In the Mahu Sag and its periphery, since discor-dogenic structures are deeply buried, the glutenite reservoirs generally present poor physical properties. The dolomitic sandstone reservoirs, with no pores in matrix, mainly exist as fracture reservoirs, but they are not large as they are ap-parently controlled by faulted belts. The volcanic reservoirs contain mostly weathered crust composed of dissolution

Wei Yanzhao et al., Effective reservoirs and hydrocarbon phases in discordogenic structures in Mahu Sag and its periphery, Junggar Basin 3

Fig. 2 Composite column of Upper Paleozoic stratigraphic sequence and source-reservoir-caprock assemblage in Mahu Sag and

its periphery

pores, and their scale is predominantly controlled by volcanic rock scale, weathering and leaching duration. For example, the large Carboniferous stratigraphic oil reservoir in the up-per side of the Kebai fault belt at the northwestern margin of the Junggar Basin, Shixi oil field and Kelameili gas field in the Luliang Uplift, all contain volcanic weathered reservoirs. Hence, presence of effective reservoirs in Carboniferous and Lower Permian in the Mahu Sag and its periphery mainly depends on the volcanic rock scale, weathering and leaching duration. Using aeromagnetic data, seismic horizon velocity data and drilling data in periphery, and considering the ab-sence of the overlying strata above Carboniferous, allowed for a comprehensive analysis of the development of vol-canic reservoirs in Carboniferous and Lower Permian in discordogenic structures in the Mahu Sag and its periphery.

2.1. Aeromagnetic anomaly feature of and volcanic rock distribution in the target layers

According to the statistics of susceptibility of Carbonif-erous cores taken from more than 60 wells in the eastern Luliang Uplift and Wucaiwan Sag in the Junggar Basin[1314], the susceptibility is (1040)×105 for sandstone and mud-stone, (8502500)×105 for basalt and diabase, (200400)× 105 for andesite and dacite, (4541000)×105 for volcanic breccia, and (120250)×105 for tuff. Sandstone and mud-stone are very low in susceptibility and can be regarded as non-magnetic. Igneous rocks are magnetic, and have differ-ent susceptibility depending on their types. Therefore, it is

feasible and practical to use magnetic data to predict igne-ous rocks and their lithologies. During the exploration of the Kelameili giant gas field in the eastern Luliang Uplift, the processing and interpretation of the 1:200000 aeromagnetic data played a major role in predicting volcanic rock distri-bution. The aeromagnetic anomaly was mainly processed by using the target optimization technique of continuation in-version vertical second derivative. The results proved to ac-curately identify volcanic rock distribution in the eastern Luliang Uplift. As a result, 85% of wells encountering vol-canic rocks are situated in high aeromagnetic anomaly re-gions. To predict the volcanic rock distribution in Carbonif-erous and Lower Permian in the Mahu Sag and its periphery, the aeromagnetic anomaly distribution maps of the Carbon-iferous and Lower Permian Jiamuhe Formation in the Mahu Sag and its periphery were obtained along with the 1:200000 aeromagnetic data in the Junggar Basin by using target optimization processing technique of continuation inversion vertical second derivative (Fig.3). Based on pe-ripheral drilling calibration and aeromagnetic prospecting experience in eastern the Luliang Uplift, the volcanic rocks mainly appear in regions with high aeromagnetic anomaly, while the sedimentary rock or volcanic sedimentary rock exists in regions with low aeromagnetic anomaly. The re-gions with high aeromagnetic anomaly in the Mahu Sag and its periphery are wide, suggesting that volcanic rock is abundant in the target layers. By overlapping the large structural trap range in Carboniferous and Lower Permian


Fig. 3 Distribution of aeromagnetic anomaly of Carboniferous and Lower Permian Jiamuhe Formation in Mahu Sag and its pe-

riphery

and the aeromagnetic anomaly map of the Mahu Sag and its periphery, it is clear that the major parts of the Northern Mahu Anticline and Well Da1 Anticline are covered in the regions with high aeromagnetic anomaly, indicating that the Carboniferous Jiamuhe Formation in the Northern Mahu Anticline and Well Da1 Anticline has abundant volcanic rock. The Middle-Northern Mahu Anticline is located in the region with high aeromagnetic anomaly, while the southern Mahu Anticline has no significant aeromagnetic anomaly, indicating that the Middle-Northern Mahu Anticline is abundant in volcanic rock, but the southern part mainly has sedimentary rocks or volcanic sedimentary rocks. The major part of the Southern Mahu Anticline has no apparent aero-magnetic anomaly, with only weak aeromagnetic anomaly in some local areas, indicating that the major part of target layers in the Southern Mahu Anticline consist of sedimen-tary rock or volcanic sedimentary rock, with only volcanic rocks in some local areas.

2.2. Seismic horizon velocity feature and weathered crust prediction

As mentioned above, the development of large-scale volcanic reservoirs is dependent on long-term weathering and leaching of the volcanic rock in addition to the scale of the distributed volcanic rock. The aeromagnetic anomaly findings suggest that large-scale volcanic rocks exist within the target layers of the Northern Mahu Anticline and Well

Da1 Anticline, as well as within some areas in the Mahu Anticline and Southern Mahu Anticline. However, the pres-ence of certain weathered crusts in the target layers of these anticlines must be analyzed to determine if there truly are large-scale volcanic reservoirs. To date, no well has been drilled to the target layers in Carboniferous and Lower Per-mian in these anticlines. The seismic horizon calibration and correlation interpretation shows that the Lower Permian formations overlap and pinch out zone by zone from west-ern margin of the Mahu Anticline to the Dabasong region. In the Dabasong region, the Lower Permian Jiamuhe For-mation is absent, and the Fengcheng Formation is thin (Fig.4). Hence, the Carboniferous formations around the Dabasong region underwent extensive weathering and leaching from late Carboniferous to early Permian. Based on the absence of the Jiamuhe Formation at that time and the partial Fengcheng Formation, it is estimated that the Carboniferous volcanic rock in the Dabasong region and its periphery generally underwent weathering for more than 10 Ma. According to the research of Zou Caineng et al.[15] on the relationship between weathering duration and the weathered crust thickness, the volcanic rock experiencing weathering and leaching for more than 10 Ma could form a weathered crust over 300 m thick. By analyzing tectonic settings, the target layers in the anticlines in the Mahu Sag and its periphery have geologic conditions for forming large-scale weathered crust.


Fig. 4 Stratigraphic section of Mahu Sag and its periphery

Fig. 5 Acoustic velocity histogram of Carboniferous volcanics in Junggar Basin before and after the weathering

In order to further verify whether there is weathered crust in the target layers in the Mahu Sag and its periphery, the changing regularity of acoustic velocity of Carboniferous volcanic rock (before and after weathering) was investigated in more than 200 wells in different parts of the Junggar Ba-sin (Fig.5). It was found that the acoustic velocity of vol-canic rocks declined to a certain extent after weathering and leaching. Particularly, the acoustic velocity of breccia tuff reflected the largest decline to about 1000 m/s after weath-ering, or even lower than that of ordinary clastic rocks. The acoustic velocity of basic volcanic rock and acidic volcanic rock declined by about 400500 m/s after weathering, and the acoustic velocity of neutral volcanic rock declined by the minimum rate – less than 30 m/s. Based on these find-ings, the velocity spectrum data of 3D seismic data covering these structures was used to calculate horizon velocity of the target layers, and then judge the possibility of weathered crust development in the target layers, according to vertical and horizontal variations of horizon velocity.

Using the calculated seismic horizon velocity data vol-

ume (in time domain) allowed for the extraction of horizon velocity-time variation curves at structural highs of the Northern Mahu Anticline, Well Da1 Anticline, Mahu Anti-cline and Southern Mahu Anticline. Low-velocity anomaly intervals were found at the Carboniferous tops in the North-ern Mahu Anticline and Mahu Anticline (Fig.6), with hori-zon velocity ranging in 51005400 m/s. Seismic horizon velocity sections through the Northern Mahu Anticline show that the low-velocity anomaly intervals in Carbonif-erous mainly distribute around the anticline top and slope belt (Fig.7). The horizon velocity curve of the Well Da1 Anticline shows that there are low-amplitude and low-velo-city anomaly intervals in Carboniferous, with the horizon velocity ranging in 49005300 m/s, which is similar to, or lower than, the velocity of the target layers in the Northern Mahu Anticline and Mahu Anticline. The horizon velocity curve of the Southern Mahu Anticline shows no low-velo-city anomaly intervals in Carboniferous, with velocity gen-erally higher than 5300 m/s. Based on seismic horizon ve-locity features of various anticlines, combined with the


Fig. 6 Seismic horizon velocity logs of Northern Mahu and Mahu Anticlines

Fig. 7 Seismic horizon velocity section of Northern Mahu Anticline

acoustic velocity range and variations of Carboniferous volcanic rock (after weathering) obtained by existing wells, it is concluded that there were large-scale weathered crust reservoirs at structural highs and slope belts of the Northern Mahu Anticline, Mahu Anticline and Well Da1 Anticline. There is no apparent low-velocity anomaly in the target lay-ers of the Southern Mahu Anticline, thus there is no weath-ered crust reservoir.

3. Hydrocarbon phases in discordogenic struc-tures in Mahu Sag and its periphery

The discordogenic structures in Carboniferous and Lower Permian in the Mahu Sag and its periphery are generally very deep. For example, major target layers in the Northern

Mahu Anticline, Mahu Anticline, Well Da1 Anticline, and Southern Mahu Anticline are more than 5500 m deep. Moreover, the volcanic reservoirs have stronger heterogene-ity. Hence, analysis of hydrocarbon phases in the target lay-ers is significant for evaluating the exploration and devel-opment potentials of the discordogenic structures. Hydro-carbon phases are decided by the type and evolution degree of organic matters in major source rocks, structural location of anticlinal traps (including burial depth of the target layers) and geothermal gradient.

3.1. Evolution of major source rocks and hydrocarbon phases in anticlinal traps

There are three sets of source rocks (the Jiamuhe Forma-tion, Fengcheng Formation and Lower Wuerhe Formation)


in Permian in the Mahu Sag. The Jiamuhe Formation source rocks are dominated by Type III kerogen to generate natural gas. With low organic carbon abundance (TOC of most source rock samples is less than 0.74%), and limited gas generation capacity (the gas generation intensity in most re-gions is less than 14×108 m3/km2), the Jiamuhe Formation source rocks contribute limited hydrocarbons to the discor-dogenic structures in Carboniferous and Lower Permian. The Lower Wuerhe Formation source rocks, located above Carboniferous and Lower Permian, don’t exist in direct contact with the underlying discordogenic structures, hence, the hydrocarbons generated could hardly make contribution to deep structures (Fig.4). The Fengcheng Formation is one of the major deep target layers, in overlaying or lateral con-tact with Carboniferous and the Jiamuhe Formation, so the hydrocarbons generated could easily migrate into and ac-cumulate in the anticlines (Fig.4). Mainly containing Type I-II1 kerogen, the Fengcheng Formation source rocks are 150 m thick on average, and feature an abundance of or-ganic carbon, reaching a stage of maturity to high maturity and suggesting a very high hydrocarbon generation capacity. Therefore, these are the major source rocks in discordogenic structures in Carboniferous and Lower Permian.

Based on analysis of bury history of the Mahu Sag, the Fengcheng Formation’s source rocks began to mature in the Middle and Late Triassic periods, and reached oil genera-tion peak in the Middle Jurassic. Currently, with an average Ro of 1.5%, it has reached gas generation peak, with small amount of oil-type cracking gas. Major parts of the Northern Mahu Anticline and Mahu Anticline, within oil generation intensity of (100-800)×104 t/km2 and gas generation inten-sity of (50-600)×107 m3/km2 of the Fengcheng Formation

source rocks, have hydrocarbon source condition for form-ing large oil and gas fields (Fig.1). Crude oil filling the Northern Mahu Anticline and Mahu Anticline in earlier stages might have been replaced by large quantities of kerogen cracking gas in later periods to form gas reservoirs. The Well Da1 Anticline and Southern Mahu Anticline, lo-cated outside but right next to the hydrocarbon generation center of the Fengcheng Formation in the Mahu Sag and Well Pen1 Sag, also have favorable hydrocarbon source conditions (Fig.1). However, the later natural gas with high maturity could not completely displace the earlier appear-ance of oil. If one ignores the possible oil cracking due to deep burial, these two anticlines must have oil and gas in coexistence.

3.2. Depth of oil cracking

Analysis using identification chart of ethane/propane ra-tio (lateral axis) and carbon isotope difference of ethane and propane (vertical axis) in natural gas samples in the Mahu Sag and its periphery[16,17] shows that the natural gas sam-ples are primarily situated in the interface among kerogen cracking gas, oil cracking gas and hydrocarbon cracking gas (Fig.8). It indicates the presence of oil cracking gas in the Mahu Sag and its periphery. This corroborates with the evolution stage of the Fengcheng Formation source rocks. Consequently, confirming the depth of oil cracking in the Mahu Sag and its periphery is of certain significance for discriminating hydrocarbon phases in deeper anticlinal traps.

According to existing simulation results, the temperature for generating oil cracking gas was 160C[1819]. Based on the third round of petroleum resource evaluation, the present geothermal gradient in the Mahu Sag and its periphery in

Fig. 8 Discrimination diagram of genetic types of oil-type gas in Mahu Sag and its periphery


the Junggar Basin is 2.3C/100 m, and the average surface temperature is 15C. Therefore, the calculated depth thresh-old for oil cracking in the Mahu Sag and its periphery is around 6200 m. Theoretically, the hydrocarbons in forma-tions below 6200 m are mainly in a gas phase. The burial depth of all target layers in the Well Da1 Anticline is less than the oil cracking depth of 6200 m, thus, no oil cracking occurred. The burial depth of major target layers in the Southern Mahu Anticline is larger than 6200 m and the oil previously filling the anticline should have cracked into natural gas.

In summary, the present hydrocarbon phase is kerogen cracking gas in the Northern Mahu Anticline and Mahu An-ticline, coexisting oil-gas in the Well Da1 Anticline, and mainly gas in the Southern Mahu Anticline.

4. Conclusions and suggestions

(1) Aeromagnetic anomalies of Carboniferous and Lower Permian Jiamuhe Formation in the Mahu Sag and its pe-riphery show that large-scale volcanic rocks exist in the tar-get layers in the Northern Mahu Anticline and Well Da1 An-ticline, and volcanic rocks exist in the middle-northern Mahu Anticline and locally in the Southern Mahu Anticline. The statistics of acoustic velocity of volcanic rock (before and after weathering) and the seismic horizon velocity fea-ture of the target layers in the anticlines reveal abnormal low-velocity belts at the Carboniferous tops in the Northern Mahu Anticline, Mahu Anticline and Well Da1 Anticline, where volcanic weathered crust reservoirs are developed. The Southern Mahu Anticline has no volcanic weathered crust reservoir.

(2) The Fengcheng Formation source rocks are the major source rocks in the discordogenic structures in the Mahu Sag and its periphery. The source rocks reached the genera-tion peak of kerogen cracking gas in the Cretaceous period. Oil cracking gas has been found in the Mahu Sag and its pe-riphery, and the depth of oil cracking is calculated at 6200 m. By comprehensive analysis, it is concluded that the pre-sent hydrocarbon phase in the Northern Mahu Anticline and Mahu Anticline is kerogen cracking gas, that of the Well Da1 Anticline is coexisting oil-gas, and that of the Southern Mahu Anticline (target layers are more than 6200 m deep) is mainly gas.

(3) The Mahu Sag and its periphery, with higher certainty of traps in deep layers, and multiple and large traps, provide superior hydrocarbon accumulation conditions and are im-portant risk exploration targets. The Northern Mahu Anti-cline and Well Da1 Anticline have favorable hydrocarbon source conditions and large-scale effective reservoirs as re-

vealed by various types of data, and the hydrocarbon phase is in the gas or coexisting oil-gas phase. Risk drilling and exploration are recommended at these two anticlines.

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Documents

Effective reservoirs and hydrocarbon phases in ... The volcanic reservoirs contain mostly weathered crust composed of dissolution . ... 4 CHINA PETROLEUM EXPLORATION Vol. 21, No. 1,