The Talmud in Kiddushin 30a states:
אמר רב ספרא משום ר' יהושע בן חנניא מאי דכתיב ושננתם לבניך אל תקרי ושננתם אלא ושלשתם לעולם ישלש אדם שנותיו שליש במקרא שליש במשנה שליש בתלמוד מי יודע כמה חיי לא צריכא ליומי
R. Safra said on the authority of R. Joshua b. Hanania: What is meant by, and thou shalt teach
them diligently [we-shinnantem] unto thy children? Read not we-shinnantem, but we-shillashtem:
[you shall divide into three]: one should always divide his years into three: [devoting] a third to
Mikra, a third to Mishnah, and a third to Talmud. Does one then know how long he will live? —
This refers only to days.
The Talmud here lists a threefold division of Torah study, from which we can infer that one fulfills his obligation with these three divisions alone. For if there were other obligatory parts of Torah study not included in these three, the Talmud would not instruct us to only split our time into three parts. And even if one were to argue that the Talmud might perhaps be speaking imprecisely and there may be other aspects of Torah study not listed, when we look at Rambam's codification of this law it is even clearer. Rambam gives precise details as to how the day breaks down; he gives as an example one who works three hours per day and studies nine hours per day. He explains that the nine hours should be divided, with three hours being spent on each area of study. Such a division clearly uses up all allocated time for Torah study; hence, if there was something else obligatory it would have to be included here.
The Talmud does not go into detail about the composition of these three divisions. However, if we turn again to Rambam's codification of this law (Hilchot Talmud Torah 1:11-12), we find it described in much greater detail:
וחייב לשלש את זמן למידתו שליש בתורה שבכתב ושליש בתורה שבעל פה ושליש יבין וישכיל אחרית דבר מראשיתו ויוציא דבר מדבר וידמה דבר לדבר ויבין במדות שהתורה נדרשת בהן עד שידע היאך הוא עיקר המדות והיאך יוציא האסור והמותר וכיוצא בהן מדברים שלמד מפי השמועה וענין זה הוא הנקרא גמרא
כיצד היה בעל אומנות והיה עוסק במלאכתו שלש שעות ביום ובתורה תשע אותן התשע קורא בשלש מהן בתורה שבכתב ובשלש בתורה שבעל פה ובשלש אחרות מתבונן בדעתו להבין דבר מדבר ודברי קבלה בכלל תורה שבכתב הן ופירושן בכלל תורה שבעל פה והענינים הנקראים פרדס בכלל הגמרא הן
A person is obligated to divide his study time in three: one third should be devoted to the Written Law; one third to the Oral Law; and one third to understanding and conceptualizing the ultimate derivation of a concept from its roots, inferring one concept from another and comparing concepts, understanding [the Torah] based on the principles of Biblical exegesis, until one appreciates the essence of those principles and how the prohibitions and the other decisions which one received according to the oral tradition can be derived using them. The latter topic is called Gemara.
How is the above expressed? A person who is a craftsman may spend three hours each day involved in his work, and [devote] nine hours to Torah study: In those nine hours, he should spend three reading the Written Law; three, the Oral Law; and three, meditating with his intellect to derive one concept from another.
The "words of the prophetic tradition" are considered part of the Written Law; and their explanation, part of the Oral Law. The matters referred to as Pardes are considered part of the Gemara.
We can apparently conclude, then, that anything that doesn't fall under this rubric is not obligatory Torah study.
(Cf. what form of study requires a blessing.)