The Veracruz Basin is located on the southwestern Gulf of Mexico and is comprised on its onshore portion by two geologic elements: The Tertiary Veracruz Basin to the east and the Cretaceous Córdoba Platform to the west, in which the Ixachi field was discovered.

The Córdoba Platform is one of the great carbonate platforms that rimmed the Gulf of Mexico Basin in the Cretaceous. It is a grossly N-S paleogeographic element that was located to the southeast of the famous Golden Lane atoll. To the southeast it extended in to the Artesa-Mundo Nuevo Platform where the giant fields of southeast Mexico were discovered in the seventies. The Cordoba Platform is a reef-rimmed shelf, covered in the Paleocene by more than 1, 000 m of basinal sediments.

In the Middle Eocene, the Laramide deformation folded and thrusted the Cordoba Platform and the Paleogene clastic sequence. Thus, two regional structural domains were formed in the platform area:

• The first domain is composed of an exposed folded and thrusted belt, which is named Sierra de Zongolica where the platform rocks are exposed, and the Buried Tectonic Front (BTF) where the platform is severely imbricated by the deformation.

• The second domain is the undeformed autochthonous part of the Cordoba platform, which due to its depth and low quality of the seismic imaging remained unexplored for a long time.

The Veracruz Basin sedimentary sequence unconformably overlies a basement formed by granitic intrusive igneous rocks, Permo - Triassic in age, 223-242 m.y. determined through the K-Ar method (Pemex, 1988).

The Upper Jurassic Pre-Kimmeridgian rocks are made up of red beds that consist of siltstones and ignimbrite layers. These red beds are in turn transitionally overlaid by a shallow platform calcareous sequence with interbedded bentonites that have been dated Late Jurassic- Kimmeridgian in age.

In the Late Jurassic Tithonian, a carbonaceous black shale sequence alternating with shaly-sandy limestones and sandy shales was deposited. The depositional environment of this section is that of an outer platform and basin. These rocks correspond to the source rock of all the basin, including the Ixachi reservoir.

The Tithonian section is transitionally overlain by Lower Cretaceous platform and basin carbonates which consist of dolomites, limestones and evaporites. In the Albian-Cenomanian, a massive platform carbonate sequence was deposited, named the Cordoba Platform. This paleogeographic element development reefal systems along its edges and lagoonal facies in its central portion. The Ixachi reservoir is found on the eastern edge of the platform.

In the Turonian, a platform was developed that interdigitates with calcareous-clayey basin facies with a high content of organic matter.

In Coniacian time, uplifting and tilting to the west took place, leading to zones of aerial exposure and erosion, and even to formation canyons in some portions of the Cordoba Platform. The eroded sediments were deposited in the slope and slope toe as debris flows as breccias and conglomerates with calcareous clasts.

In the Paleogene, as the Laramide Orogeny began to deform the carbonate platform, foreland sequences made up by an alternating shales, sandstones and conglomerates, were accumulated in a basinal environment as debris flows, in fan and channels. The composition of this clastic section indicates that sediments´ source were of igneous and metamorphic origin, such as at the Alto de Santa Ana, La Mixtequita and Chiapas Massifs, and the Sierra de Juárez. The presence of carbonate clasts is evidence that the Córdoba Platform also provided sediments to this Paleogene section. This Paleogene section is considered the vertical seal of the Ixachi reservoir, since only the Paleocene reaches more than 1,000 m in thickness.

The Neogene section is made up by alternating shales and sandstones deposited in channel complexes of basinal submarine fans. The Miocene is the main stratigraphic level due to it contains the main gas reservoirs of the Veracruz Basin.

The autochthonous portion of the Córdoba Platform in the center of the basin is located at a depth of more than 6 kms, so early seismic imaging was of poor quality, causing it to remain unexplored. With improved seismic technologies, the autochthonous domain began to be well visualized. Hence, today we know that the autochthonous domain consists of reefs and calcareous sand banks, that represent a frontier play covering an area of 14,500 km2.

In 2017, the Ixachi-1 well was successfully completed, discovering a reservoir with gas and 41° API condensate in Lower Cretaceous reefal limestones and carbonate sand banks with microvugs and fractures.

The top of the reservoir in the Ixachi-1 well was found at a depth of 6,520 vertical meters; and for the purpose of a 3P estimation, the conventional base of the reservoir was established at a depth of 7,300 vertical meters, with an area of 34 km2. The initial well production reached 3,269 barrels per day of condensate and 29 mmcfd of gas through a choke size of ½ “. The original pressure was of 17,200 psi in the reservoir and of 11,200 psi at the wellhead. Average porosity is 5.7 %, Sw is13% and temperature is168° C. Initial estimated 3P reserves based on the Ixachi-1 well data only was of 366 MMBOE.

In 2018, the reservoir was appraised by the Ixachi-1DEL and Ixachi-1001 wells. These wells confirmed the extension of the reservoir to the north and south, increasing thereby the 3P area to 75 km2. In addition, the Ixachi-1001 well showed through an MDT that the conventional limit extends to a depth of 7,600 vertical meters, increasing net thickness to more than 1,200 m. Therefore, current 3P reserves for the Ixachi reservoir have been reassessed at 1,314 MMboe with an original volume in place of 8,206 MMMcf of gas and 1,002 MMbls of condensate, with a current recovery factor of 0.07 % for gas and condensate, a condensate-gas ratio of 130.1 bls/mmcf, a saturation pressure of 592 kg/cm2 and a permeability range from 0.66 to 7.5 mD.

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