RESEARCH PAPER

PETROLEUM EXPLORATION AND DEVELOPMENT Volume 40, Issue 5, October 2013 Online English edition of the Chinese language journal

Cite this article as: PETROL. EXPLOR. DEVELOP., 2013, 40(5): 592–598.

Received date: 06 Nov. 2012; Revised date: 05 Jul. 2013. * Corresponding author. E-mail: quhz555@yahoo.com Foundation item: Supported by the National Science and Technology Major Project (2011ZX05004-004). Copyright © 2013, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.

Karstification of reef-bank facies carbonate and its control on pore distribution: A case study of Upper Ordovician Lianglitage Formation in eastern Tazhong area, Tarim Basin, NW China

QU Haizhou1,2,*, WANG Zhenyu1,2, YANG Haijun3, ZHANG Yunfeng1,2, YU Hongfeng3, WANG Xi3 1. State Key Laboratory of oil and gas reservoir geology and exploitation, Southwest Petroleum University, Chengdu 610500, China 2. Institute of Resources and Environment, Southwest Petroleum University, Chengdu 610500, China 3. PetroChina Tarim Oilfield Company, Korla 841000, China

Abstract: The karstification type and characteristics and its control on pore distribution in reef-bank carbonate rock in the Upper Ordo-vician Lianglitage Formation (O3l) in eastern Tazhong area were studied using cores, thin sections and well logging data. The area ex-perienced two types of karstification: syndepositional karstification and early diagenetic karstification near surface. During the depositional stage of reef-bank complex, meteoric diagenetic lens with lots of selective dissolution pores were formed by the syndepositional karstifi-cation, they were developed in the middle and upper parts of reef-bank sedimentary cycles, and a single porous/cavity layer has a thick-ness of 10-30 m, increasing the absolute porosity by 3%-4%. After depositional stage, sediments were uplifted and exposed without ex-periencing burial diagenesis, the depositional topography was influenced by early diagenetic karstification near surface and turned into karst paleotopography and formed four karstification zones within 100 m below the top of the Upper Lianglitage Formation. The surface karstification zone and radial flow karsitifiction zone have favorable porous connectivity, increasing the absolute porosity by 4%-8%. These two types of karsitifiction are continuous in diagenesis stage and the formed porosity is successive in distribution. The favorable space-time coupling is the key reason for effective pores being largely developed in the O3l3-O3l1 members.

Key words: pore distribution; karstification; reef-bank facies; Lianglitage Formation; Ordovician; eastern Tazhong area

1 Introduction

Generally, carbonate karstification occurs in atmospheric water environment and buried environment[1−4]. By stratigra-phy, James divided paleokarst in atmospheric water environ-ment into three types: sedimentary paleokarst, local paleokarst and regional paleokarst[5]. Sedimentary palaeokarst means the selective dissolution of unstable minerals mainly by atmos-pheric water in the depositional stage when the sea level change and quick deposition resulted in short-term expo-sure[6−8]. Regional paleokarst means the karstification of ex-tensive fractures/cavities and regional unconformity as se-quence boundary that were formed because the sediments exposed to atmospheric environment the epidiagenetic phase after middle to late diagenetic stage[9]. Local paleokarst, gen-erated in the stage between above two types, means the karsti-fication of carbonate platform exposed to the atmospheric environment due to the syndepositional tectonic movement; its extent varies greatly with exposure time. In China, the re-

searchers often classify and name karstifications by diagenetic stage. For instance, the syngenetic karst and weathered crust karst correspond to the sedimentary paleokarst and regional paleokarst respectively[6−9]. Karstification in an open atmos-pheric water environment is most beneficial for pores, and in proper combination with sedimentary facies, dolomitization, buried denudation or tectonic disruption, forms a set of fa-vorable carbonate reservoirs[10−18]. However, influenced by the discontinuity of diagenetic stage and buried diagenetic trans-formation, no study has been made on the conjunction be-tween two karstifications in atmospheric water environment (syngenetic karst and weathered crust karst) and the pore de-velopment and distribution thereof.

Tazhong No.1 fault formed in the late stage of Early Ordo-vician, as a result of strong thrusting, forming a north-dipping slope-break zone after collapse and denudation[19], in NW-SE trend (Fig.1). During the deposition of the Late Ordovician Lianglitage Formation, the palaeoclimate was warm, sea level rose steadily, and good circulation condition of sedimentary

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Fig. 1 Structural location of the study area

water and normal salinity existed in Tazhong-Bachu platform. In the fault slope-break zone, the water was shallow and the wave action was strong, providing conducive conditions for organisms to breed; carbonate productivity was high, building the composite sedimentary formation of reef and grain-bank, i.e. reef-bank complex[20−21]. In the Tarim basin, the Liangli-tage Formation of Upper Ordovician contacted with the over-lying Sangtamu formation in regional conformity [22]. In this period, the palaeokarst could not be called “local paleokarst”, neither “regional paleokarst” since it formed in the period of compressive uplifting and exposure after the sedimentary period, not an epidiagenetic stage. Therefore, considering the stage and range of the karstification, the authors defined it as the early-diagenetic near-surface karst. In view of the special geology in the study area, the paper discusses the karstifica-tion of the upper Ordovician Lianglitage Formation in atmos-pheric water environment and its control on the pore distribu-tion.

2 Karstification in the study area

In the study area, the Lianglitage reef-bank facies carbonate rock suffered from syngenetic karstification and early-dia-genetic near-surface karstification. The former is the process that, in the syngenetic (penecontemporaneous) stage, the reef-bank complex with positive geomorphic characteristics exposed in the atmospheric environment to form the atmos-pheric diagenetic lens, and the unstable minerals were selec-tively denudated due to leaching and penetration by unsatu-rated atmospheric fresh water. Whereas, the later is a process that, after the sedimentary period, the sediments were denu-dated and filled non-selectively and extensively by the at-mospheric water when they were uplifted and exposed in the atmospheric environment, without undergoing the buried dia-genetic environment in the middle-late diagenetic stage.

2.1 Atmospheric fresh water diagenetic lens

In the sedimentary period, the organic reef and grain bank sediments deposited faster and thicker than the interbank sea

sediment, with higher sedimentary topography. Once the sea level fell off, the island reef-bank complex exposed to the atmosphere and then formed atmospheric fresh water diage-netic lens with internal zoning, which would denudate the aragonite, high-Mg calcite and other unstable minerals, gen-erating a large number of selective dissolved pores.

2.1.1 Denudation and cementation characteristics

Atmospheric fresh water diagenetic lens can be divided into vadose zone (above the free surface) and underflow zone (under the free surface). Vadose zone is an open system in the aerosphere, in which the pores contain both atmospheric water and air. The atmospheric water with unsaturated carbonate minerals, during flow, selectively dissolves the instable min-erals (e.g. aragonite and high-Mg calcite) between or in grains or in organism cavity, thereby intergranular dissolved pore, intragranular dissolved pore and moldic pore were formed (Fig.2a-2c). Pore water with dissolved carbonate minerals flows into the grains, and then form crescent-shaped or drape-shaped calcite cements under the surface tension and gravity (Fig.2d). Pore water with silt sized clastic sediments, flowing at high rate, fills the lower part of dissolved pore/cavity, forming a geopetal structure with the sparry cal-cite at the upper part (Fig.2e). In contrast to the lower part of underflow zone, the fluid in the upper part contains higher CO2 (near the free surface), the pore water is unsaturated, and the quasi-steady carbonate components are more easily dis-solved, forming a dissolved pore zone. The CaCO3 dissolved in the pore water may appear in typical diagenetic state (e.g. syntaxial overgrowth) around the echinoderm clastics (Fig.2f).

2.1.2 Vertical distribution characteristics

About 360m cores are taken from 25 wells in the Liangli-tage Formation in Tazhong area, and 300 thin sections are made. It is observed that thickness of atmospheric water lens in each reef-bank sedimentary cycle is 10−30 m, and multi-phase lens develop vertically and mostly in the Liang-3 ~ Liang-1 members, the main structures of the reef-bank com-plex. In the depositional stage of Liang-3 ~ Liang-1 members, Lianglitage Formation, Late Ordovician, the eastern Tazhong No.1 slope break zone was in aggradation (or progradation) parasequence set of highstand system tract, with the accom-modation space of sediments increased, and the very thick reef-bank complex deposited in 3~5 stages. A single reef-bank sedimentary cycle, with thickness up to 50−100 m, repre-sented as grain-bank sediments such as arene bank and bio-clastic bank that was developed on the organic reef built up by framework stone, bafflestone and bindstone. Due to the rapid reef-bank construction and sea-level fluctuation, once the complex is exposed to atmospheric environment, it will be dissolved by the atmospheric fresh water, to form a large number of selective dissolved pores/cavities, such as inter-granular dissolved pore, intragranular dissolved pore, moldic pore, and organism cavity pore. Accordingly, the atmospheric fresh water diagenetic lens is generated.

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Fig. 2 Single polar casting thin sections of the Lianglitage reef-bank carbonate rock in eastern Tazhong Area

2.2 Early diagenetic near-surface karstification zones

With thin sections, several diagenetic features are often ob-served, e.g. less dislocated particles, less-frequent point-con-tact between particles, and few linear (or cambered) contacts. It indicates that the reef-bank complex did not experience compaction under buried environment and other diagenesis; instead, it was uplifted and exposed to the warm and humid near-surface atmospheric environment in the early diagenetic stage which is characterized by weak cementation and high primary porosity in early stage, and as a result of leaching,

erosion, corrosion and filling by atmospheric water, non-selective dissolved pore, cave and fractures that was semi-filled or filled by gray green muds and breccias, in cer-tain zoning vertically. The authors name the karstification associated with unconformity surface but different from re-gional and local palaeokarsts as early diagenetic near-surface karstification.

2.2.1 Dissolution and filling

Early diagenetic near-surface karstification zone, from top to bottom, can be divided into surface karst zone, vertical

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percolation karst zone, runoff karst zone and deep slack water dissolution zone. Each zone shows different porosity types and dissolution/filling characteristics. The surface karst zone, in the upper part of vadose zone, exposes directly to the at-mosphere; driven by CO2-rich atmospheric waters and surface runoff, this zone experiences intense karstification (Fig.3a), with small dissolution space and good connectivity. Vertical percolation karst zone, in the lower part of vadose zone, rep-resents high-angle dissolved fractures and caves (Fig.3b and 3c) under the action of vertical flow and corrosion of at-mospheric water; in this zone, lateral flow of the atmospheric water is weak, so lateral connectivity between fractures/caves is poor. Runoff karst zone is mainly located in the saturated zone below the free surface, where a series of nearly-hori-zontal dissolved pores, caves or fractures usually occur, since the ground water flows in a nearly-horizontal way along the fractures or primary pores (Fig.3d); the dissolution space is large and connectivity is good in this zone. In the deep slack water zone, the groundwater contains highly-saturated car-bonate minerals, leading to slow flow and weak dissolution.

2.2.2 Vertical distribution

Taking Well TZ62 in Fig.4 as an example, based on core observation and image/conventional logging, the surface karst zone, vertical percolation karst zone, and runoff karst zone are identified, with thickness of 30.00 m, 29.00 m and 16.00 m

respectively. Deep slack water dissolution zone is not pene-trated in this well. The surface karst zone at 4 685.00− 4 715.00 m corresponds to high natural gamma and shows dark spots and dark groups on the image logging map, repre-senting as dissolution pores and holes. The core proves that this section contains dissolved caves semi-filled or filled with breccias and mud. For vertical percolation karst zone at 4 715.00−4 744.00 m, sinusoidal or "V"-type dark stripes/ bands, dark/bright spots are observed on the image logging map. The core demonstrates high-angle dissolved fractures and karrens semi-filled and filled with mud and breccias (Fig.3b and 3c). The runoff karst zone at 4 744.00−4 760.00 m is characterized by near-horizontal dark low-resistivity bands/stripe, prolate dark spots and dark spots groups on the image logging map; the core shows encapsulated dissolved pores/caves distributed along stratums, and also near-horizon-tal dissolved fractures and mesh fractures which are filled with gray-green mud and breccias, and the core is partially broken. The pores, holes and fractures are developed obvi-ously in this zone, and the connectivity is also better.

3 Pore distribution

The pores are concentrated in Liang-3 ~ Liang-1 members in the study area, with the following characteristics. Firstly, the pores are diversified, such as intergranular/intragranular dissolved pores, moldic pores, biological cavity pores and

Fig. 3 Early diagenetic near-surface karstification of the Lianglitage reef-bank carbonate in the eastern Tazhong Area

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other selective dissolved pores coexisting with other non- selective dissolved pores such as dissolved fractures/trench/ caves. Secondly, the crescent and drape-shaped calcite gener-ated due to chemical precipitation coexist with the fillings formed by mechanical action during exposure such as gray-green mud and breccia. Thirdly, the pores are effective. In particular, the selective dissolved pores generated by syn-genetic karstification did not experience the middle-late diagenesis, and dissolved and stacked selectively by early diagenetic near-surface karsts when compaction and cementa-tion were weak. They are moderately filled and conserved well generally.

In the Lianglitage Formation, the pores are distributed ver-tically as follows: selective dissolved pores developed in the middle-top reef-bank sedimentary cycle, and multicycle reef-bank sedimentation resulted in many sets of pore/cavity layers in the Liang 3 – Liang 1 members, with single-layer thickness of about 10−30 m. Early diagenetic near-surface karstification happened in the reef-bank within 100m to top of Lianglitage Formation, with best pores and connectivity de-veloped in the upper surface karst zone and the runoff karst zone below the free surface. According to the core and section observation, the atmospheric diagenetic lenses, which can increase absolute porosity by 3% to 4%, and early diagenetic

near-surface karst, which can increase it by 4% to 8%, are superimposed to form major effective pores in the reef-bank carbonates. This is the key factor to develop favorable reser-voirs in deep Lianglitage Formation of the Tazhong area. In Fig.4, the aggradation parasequence set in Well TZ62 reaches 62 m. It is found with the core and log data that early diagenetic near-surface karstification plays positive role in development of macroscopic pores in 4 685.00−4 715.00 m and 4 744.00−4 760.00 m. In this well, total thickness of ef-fective reservoirs is 68.80 m, as 91.00% of total formation thickness. Class I and II favorable reservoirs are predominant, with thickness of 48.40 m, as 64.20% of total formation thick-ness; Class III reservoir is 20.40 m thick, as 26.80% of total formation thickness. Especially in 4 744.00−4 760.00 m, the atmospheric diagenetic lens and runoff karst zone are super-imposed, both with well-developed pores, forming the thick high-quality reservoirs, and interlayers are not developed.

4 Control of the karstification on pore distribution

In the study area, two types of karstification in the atmos-pheric water environment in the Lianglitage Formation are closely related to the sedimentary facies and the structural evolution. They have continuity in diagenetic stage, and good

Fig. 4 Pore development and distribution in the Upper Ordovician Lianglitage Formation in Well TZ62 (Rd- deep resistivity; Rs-shallow resistivity).

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inheritance in space. In view of diagenetic stage, after the ingression in the Tazhong area, the depositional system of open platform - platform edge – slope - basin had been built; when Liang 3 - Liang 1 members deposited, the highstand system tract at the platform edge was dominated by stacking of aggradation parasequence set, so the very thick organic reef-grain bank complex with obvious uplift was accumulated. The sea level fluctuated frequently in the depositional stage, and the reef-bank with high relief was easily exposed, as is-land, in the atmospheric water environment; in the middle and upper parts of each stage of reef-bank complex, atmospheric fresh water diagenetic lens were formed, and the syngenetic karstification caused selective dissolution, leading to pore/cavity intervals (Fig.5a). After the depositional stage, the reef-bank complex did not experience such diagenesis as compaction at the burial stage, but was uplifted to expose and selectively dissolved and filled with the early diagenetic near-surface karst when the cementation was weaker and the early pores were highly effective. Hence, the two karstifica-tions are time-continuous. In view of the distribution space, due to the difference of depositional rate, few or decade of

meters of depositional physiognomy difference may exist between the reef-bank complex in the rimmed platform sys-tem and the interbank sea, and the pores/caves formed as a result of selective dissolution by syngenetic karst distributed in the middle-upper part of the complex; with the uplift and exposure after the depositional stage, the high-relief reef-bank complex transformed into high position of the karst palaeo-geomorphology, while the interbank sea deposited in the low position of the karst palaeogeomorphology, both inherited in space. The reef-bank rocks at high position captured the at-mospheric precipitation and then formed the fractures/cavities with high validity and connectivity in the the surface karst zone and the runoff karst zone below free surface (Fig.5b). Hence, both karstifications were continuous in diagenetic stage, with inherited pore distribution. This favorable space-time coupling is critical for the effective pore distribu-tion and favorable reservoirs in Liang 3 – Liang 1 members in the study area.

5 Conclusions

The reef-bank carbonates of the Lianglitage Formation of

Fig. 5 Genetic Model of Pore Distribution in the Reef-bank Carbonates of Lianglitage Formation in the Eastern Tazhong Area

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Upper Ordovician in the eastern Tazhong area were suffered from two karstifications – syngenetic karstification and early diagenetic near-surface karstification. The former produced the atmospheric fresh water diagenetic lens with considerable dissolved pores in the middle-upper part of each reef-bank sedimentary cycle, resulting in multiple sets of early pore/cavity layers in Liang 3 – Liang 1 members, 10−30 m thick each. The later happened within 100 m to the top of Lianglitage Formation, with best pores and connectivity de-veloped in the upper surface karst zone and the runoff karst zone below the free surface. The atmospheric diagenetic lenses, which can increase absolute porosity by 3% to 4%, and early diagenetic near-surface karst, which can increase it by 4% to 8%, are superimposed to form major effective pores in the reef-bank carbonates. Both karstifications were continuous in diagenetic stage, with inherited pore distribution; this fa-vorable space-time coupling is critical for the effective pore distribution and favorable reservoirs in Liang 3 – Liang 1 members in the study area.

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