Paleoenvironments and reservoir characteristics of Paleogene strata, Southwest Tarim Basin

Abstract

Paleogene strata in Southwest Tarim Basin are important hosts of hydrocarbons with the highest quality reservoirs, a product of depositional environment, diagenesis and tectonics. The Paleogene interval is composed, in ascending order, of the Altash, Zimugen, Karatar, Wulagen and Bashbluk formations. Five facies associations are interpreted, respectively, as gypsum bay, alluvial fan-braided river, intertidal flat, supratidal flat and restricted platform. Stratigraphic evolution of Paleogene is recorded in vertical changes of sedimentary environment and climate. At least four transgressive–regressive cycles are recognized that can be correlated with the global eustatic changes. Detailed paleoenvironmental analysis laid a foundation for predicting the reservoir rocks in this mixed clastic–carbonate sequence. Braided channel sandstone and restricted platform grainstone constitute the best reservoirs in Paleogene. The most important diagenetic processes in sandstones that resulted in porosity and permeability changes are mechanical compaction, cementation, and dissolution of unstable clastic grains and cements, whereas in carbonate rocks, the dominant factors are cementation and dissolution. Tectonic fracturing developed in multiple stages in Paleogene has played an important role in improving the quality of reservoir rocks. Stratigraphic evolution of the Paleogene resulted in stacking of seal rocks above reservoirs during the late transgressive and regressive phases of sedimentation.

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Carbonates and Evaporites (2023) 38:76
https://doi.org/10.1007/s13146-023-00902-9
ORIGINAL ARTICLE
Paleoenvironments andreservoir characteristics ofPaleogene strata,
Southwest Tarim Basin
FengGuo1· YankaiZhu1· XiaoxiaPeng2· QiPan1· WenfangYuan3· ZhilinYang3
Accepted: 23 September 2023 / Published online: 20 October 2023
© The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2023
Abstract
Paleogene strata in Southwest Tarim Basin are important hosts of hydrocarbons with the highest quality reservoirs, a product
of depositional environment, diagenesis and tectonics. The Paleogene interval is composed, in ascending order, of the Altash,
Zimugen, Karatar, Wulagen and Bashbluk formations. Five facies associations are interpreted, respectively, as gypsum
bay, alluvial fan-braided river, intertidal flat, supratidal flat and restricted platform. Stratigraphic evolution of Paleogene is
recorded in vertical changes of sedimentary environment and climate. At least four transgressive–regressive cycles are rec-
ognized that can be correlated with the global eustatic changes. Detailed paleoenvironmental analysis laid a foundation for
predicting the reservoir rocks in this mixed clastic–carbonate sequence. Braided channel sandstone and restricted platform
grainstone constitute the best reservoirs in Paleogene. The most important diagenetic processes in sandstones that resulted
in porosity and permeability changes are mechanical compaction, cementation, and dissolution of unstable clastic grains
and cements, whereas in carbonate rocks, the dominant factors are cementation and dissolution. Tectonic fracturing devel-
oped in multiple stages in Paleogene has played an important role in improving the quality of reservoir rocks. Stratigraphic
evolution of the Paleogene resulted in stacking of seal rocks above reservoirs during the late transgressive and regressive
phases of sedimentation.
Keywords Paleoenvironments· Reservoir characteristics· Paleogene· Tarim Basin
Introduction
Paleocene, Eocene and Oligocene marine sedimentary strata
are exposed in the southwestern Tarim Basin, which is one
of only three Paleocene marine strata outcrops in China, and
are well preserved. The southwest depression is an important
strategic succession for oil and gas exploration in the Tarim
Basin (Jin etal. 2000; Sun etal. 2005; Yang etal. 2012; Yue
etal. 2017).
Since the 1950s, the stratigraphy, paleontology, sedimen-
tation and tectonics of the Tarim Basin have been studied
to varying degrees, and the stratigraphic system and sedi-
mentary framework of the Paleogene were initially estab-
lished in order to cooperate with the oil and gas resource
investigation and potassium resource (Hao etal. 1979; Yong
and Dan 1986; Tang etal. 1989; Guo 1990a, b; Wang etal.
1996). The Paleogene in Southwest Tarim Basin was a time
of restricted circulation dominated by bays and tidal flats
until, in the Neogene, an environmental change occurred
from sea to land (Yong and Dan 1986; Jia etal. 2002; Fang
etal. 2009; Zhang etal. 2021). In Paleogene, Tethys seawa-
ter invaded eastward into Tarim basin many times through
Alai Strait in the west. The sediment was mainly sourced
from the Tianshan Mountain in the north and Kunlun Moun-
tain in the south, and the depocenter was located to the west
of the present Southwest Tarim Basin (Hao etal. 1984; Fang
etal. 2009; Xi etal. 2020). The southwest depression of
Tarim basin is opened to the west, which constitutes the
eastern bay of the continental surface sea (Tajik Sea) on the
northern margin of the Tethys Ocean. The mixed deposition
* Feng Guo
guofeng@xsyu.edu.cn
* Xiaoxia Peng
pengxx2002@sina.com
1 School ofEarth Sciences andEngineering, Xi’an Shiyou
University, Xi’an710065, China
2 Department ofGeology, Northwest University,
Xi’an710069, China
3 China National Petroleum Corporation, Exploration
andDevelopment Research Institute ofTarim Oilfield
Company, Korla841000, China
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